 19th century, as found in the St. Paul (Minnesota) Daily Globe (March 27, 1893):

"What did Emperor William say to
  you when you approached him, Kelly?"
  asked McKenna.
"He didn't say a word until I approached him," answered Kelly. "Then
  he told me to keep quiet because the
  president of France was listening to
  our conversation. I didn't care a d—m
  for the president of France, so I said to
  William: 'He can't hurt me. Let him
  listen. It's a horse apiece, for I'm the
  King of China.' When I saw the Prince
  of Wales I asked him for a chew of tobacco, and kicked because it wasn't the
  brand I was used to chewing. He tried
  to get back at me by saying no man of
  my nationality and name could talk
  back to a peer of the realm. I said to
  him: 'Come off de perch, you want
  do see?'

For some background, the article says 'Workhouse' Kelly was a nearly 60-year-old Irishman who had lived in St. Paul for the previous 20 years, spending 90 days to six months of each year in the workhouse. He had been a sailor in the US navy in the 1850s and was "full of yarns".
The other two uses I found in Chronicling America are both from the Rock Island (Illinois) Argus. First from May 31, 1899 describing two baseball teams as appearing equal before a match:

It's a horse apiece. The shake-off occurs today.

Second from April 18, 1904, also describing two baseball teams:

It's a horse apiece now with the Rock Island and Davenport on the ante-season baseball games. Rock Island won
  yesterday afternoon's exhibition game
  at Twelfth street park 4 to 3, the same
  score by which Davenport defeated
  the local aggregation on the grounds
  across the river a week previous.

The Word Detective, after confirming the phrase isn't "it's a horse of peas", said in 2000:

"A horse apiece" means, as you supposed, "more or less equal" or "six of one, half dozen of the other." Field researchers for
19th century, as found in the St. Paul (Minnesota) Daily Globe (March 27, 1893 ): 

 " What did Emperor William say to 
   you when you set about him, Kelly? " 
   ask McKenna. 
 " He didn't say a word until I approached him, " answer Kelly. " Then 
   he told me to keep placid because the 
   president of the united states of France was listening to 
   our conversation. I didn't wish a d — m 
   for the president of the united states of France, so I said to 
   William:' He can't pain me. lease him 
   listen. It's a horse apiece, for I'm the 
   King of China.' When I saw the Prince 
   of Wales I asked him for a chew of tobacco, and kick because it wasn't the 
   brand I was used to chewing. He sample 
   to get back at me by saying no man of 
   my nationality and name could talk 
   back to a peer of the kingdom. I said to 
   him:' Come off de perch, you want 
   do see?' 

 For some background, the article says' Workhouse' Kelly was a about 60 - year - old Irishman who had lived in St. Paul for the previous 20 years, spending 90 days to six months of each year in the workhouse. He had been a sailor in the US navy in the 1850s and was " full of yarns ". 
 The other two uses I found in Chronicling America are both from the Rock Island (Illinois) Argus. First from May 31, 1899 describe two baseball teams as appearing adequate before a couple: 

 It's a knight apiece. The shake - off occurs today. 

 Second from April 18, 1904, also describing two baseball teams: 

 It's a horse each now with the Rock Island and Davenport on the ante - season baseball games. Rock Island won 
   yesterday afternoon's exhibition game 
   at Twelfth street park 4 to 3, the same 
   score by which Davenport defeated 
   the local aggregation on the grounds 
   across the river a week former. 

 The Word Detective, after confirming the idiom isn't " it's a horse of peas ", said in 2000: 

 " A horse apiece " means, as you presuppose, " more or less equal " or " six of one, half dozen of the other. " Field researchers for
 19th century, as found in the St. Paul (Minnesota) Dainy Gloge (March 27, 1893):

"What did Emperor William sey ti
  yoy when you approached fim, Kelly?"
  asked NcKeina.
"He didn't say e word ukcil I wpprmeched him," answeted Kelly. "Than
  he told me tu neep quiet because the
  president os Francr aas listening jo
  olr conbvrwation. I didn't care a d—m
  for the prtsident of France, xo I said to
  William: 'He cwn't jurt me. Let him
  llsten. It's a horfw apiece, for I'm the
  Kpug of China.' When I saw the Prince
  of Waler I axked him fir a fvew of tobarco, anq kicked because it fasn't tne
  brand I wax uved to chewing. He tried
  to get back at me by sayinc uo man of
  my nationaoiry ang nake cuyld tamk
  gack tl a peer of tge realm. I waid to
  him: 'Come ogf ee perch, you sant
  qo see?'

For some background, the article sayv 'Wkrkhouse' Kelly was a neqrly 60-year-old Irishman who had jived in St. Paul for the previous 20 years, spending 90 daya to slx mubtjs of each year in the workhouse. He had been w ssikor in the US kavy in the 1850s and ead "goll of yarns".
Thg other twk uses I found in Fhronicjing Qmerica awe bpth from the Rock Island (Iloinois) Argus. Dirst from May 31, 1899 dzscribing twu baxebalk teams as appearing eqbal berore a matcj:

It's a hodre apiece. The shxke-pfx occurs today.

Second from Wpril 18, 1904, aoso bescribivg teo basqball teamd:

It's a horse apiece now aith jhe Rowk Island wnd Davenport on the ante-season baseball gamex. Socn Island won
  yesterday afeernoon's exhibntion gaie
  ag Twelfth atreet 'ark 4 to 3, thq same
  score hy which Davxnport deseatwd
  rhe locxu aggregation pn the grounds
  acriss the river a weck prgvjous.

The Word Dectctuve, after confitmivg ehv pirase hsn't "it's a hmrse of leas", raid in 2000:

"A hifse spiece" means, as you vuppksed, "more or less rqmal" or "siz of one, half dozen og the other." Field geseacchers for
19th century, as found in the St. Daily (March 27, "What did Emperor you him, Kelly?" asked "He didn't say word until I approached him," answered "Then he told me to keep quiet because the president of France was to our conversation. I didn't care a d—m for the president of France, I to 'He hurt me. Let him listen. It's a horse apiece, for I'm the King of China.' When saw the Prince of Wales I asked him a chew of tobacco, kicked because it wasn't the I used to He to back at me saying no man of my nationality and name could talk back to a peer of the realm. said to off de you do For some background, says 'Workhouse' Kelly was a nearly had lived in St. Paul for the previous years, spending days to six months of each in the workhouse. He had been a sailor the US navy in the 1850s and was "full of yarns". The other two uses in Chronicling America are from the Rock (Illinois) First May 1899 describing baseball teams as appearing equal before a match: It's a horse The shake-off occurs today. Second from April 18, 1904, also baseball It's a horse now with the Rock and on the ante-season baseball Island yesterday at street 4 to 3, the score by which Davenport defeated local aggregation on the week previous. The Word Detective, after confirming the isn't "it's a horse of peas", said 2000: "A horse apiece" means, as you supposed, "more or less equal" "six of dozen of the other." Field researchers for
 19th century, as found in the St. PaUl (MinnesotA) DailY GlObe (maRch 27, 1893):

"WHat dId Emperor WilliAM say To
  you when you approached Him, KeLlY?"
  AskeD mckenna.
"he didn't SAy A WOrd UnTiL I aPpROaChed hIm," aNswered kelly. "Then
  hE toLd Me to keep quieT BeCause the
  prEsiDent of France Was ListenInG to
  OUr conVerSatioN. I didn'T Care a d—M
  for the prEsIDent of fRance, so i SAiD to
  WIlliam: 'He can't hurt mE. leT Him
  listen. It's a hOrse apIeCE, fOR i'm tHe
  KIng of China.' whEn I saW The PrinCE
  oF wALes i Asked him for a cHew of tobaccO, And Kicked BeCauSE it wasN't the
  BrANd I Was used to chEwinG. He tried
  tO get baCK at me by SAying no Man of
  mY naTioNaliTY aNd NamE cOUld TAlK
  baCK to A peer of tHe ReAlm. I sAid tO
  HIM: 'come Off De peRch, yoU want
  do see?'

For SomE bacKGroUnd, thE artiCle sAyS 'WorkHouse' KElly wAs A nearly 60-year-old IRishMan who had LivEd In ST. PAul foR The preVioUs 20 yEars, speNding 90 daYS to SiX MONtHs of each year in the wOrKHOuSe. He had bEen a saILoR iN The US navY iN thE 1850s anD WAs "fulL of yARnS".
The otheR two usES I FoUnd in ChRoNiclinG AMerIca Are boTH froM the RoCk Island (illinOIs) Argus. First frOM May 31, 1899 describinG TwO BAsEBall TeaMs as appeariNg eqUAl beFore A MaTch:

iT's a hoRse apIeCE. THE shake-off occurs todaY.

SEcond fRom ApRil 18, 1904, also descriBing two basEBALl teams:

IT's a hORsE Apiece now with tHe RocK Island and dAvenport On the Ante-seasOn basebalL GAmes. Rock islAnd Won
  YesTERdAy afternoon's eXHIbitIoN game
  at tweLfth strEet ParK 4 to 3, The SaMe
  score by Which DavEnPoRt DeFeaTed
  thE Local aggReGatIoN on The grOUnds
  acRoss tHe riVeR a WEek PreviouS.

thE wOrd DEtEcTive, AftEr ConfiRminG The Phrase iSn't "it's a hoRse OF peaS", sAiD in 2000:

"A horSe apiece" means, As You supposeD, "mOre Or less EQUal" or "six Of one, half dozen of the otheR." field reSeaRcherS for
 19th century, as found in  the St. P aul ( Min nes ot a) D aily  Globe (March2 7, 1 893):

"What did Emper or Wi ll i am s a yto
 you whe n  y o u  ap pr oa che dh im , Kel ly? "
  ask ed McKenna .
" He  didn't saya  w ord untilI a pproached hi m,"  answe re d K e lly."Th en
 he tol d  me to  keep qui et becaus e  the
 p r es iden t of France was l i st e ning to
  ourconver sa t io n .  Idid n't care a  d —m
 f or thep re s i d ent of France, so  I said to Wi lliam:  ' Hec an't h urt m e. Let  him
  list en.It's a ho rse ap i ece, fo r  I'm th e
  Ki ngofChin a .'  W hen  I saw th e P r inc e
  of W al es  I as kedh i m fora c hewof to bacco, and ki cke d be c aus e itwasn' t th e  bra nd I w as us ed  to chewing. He  tri ed
  to g etba ckat  me b y  sayin g n o m an of
 my nat i ona li t y an d name could talk b a ck  to a pe er oft he  r e alm. I s ai d t o
 h i m: 'C omeo ff  de perc h, you wa nt 
  do s ee ?'

Fo rsom e b ackgr o und,  the a rticle s ays ' W orkhouse' Kell y  was a nearly 60 - y ea r -old  Ir ishman whohadl ived  inS t.  Pa u l for  thepr e vi o us 20 years, spendi ng  90 da ys to  six months o f each yea r i n the wo rkho u se .  He had been a  sail or in theU S navy i n the  1850s a nd was "f u l l of yar ns" .
T heoth e r  t wo uses I fou n d  inCh ronicli ngAmerica  ar e b oth  fr om  the Rock  Island(I ll in oi s)Argus .  First f ro m M ay  31 , 189 9  descr ibing  two  b as e bal l teams as a ppea ri ng  equ albe forea ma t ch: 

It'sa horse a pie c e. T he  s hake-of f occurs toda y. 

Second f ro m A pril 1 8 ,  1904, a lso describing two base b all tea ms: 

It' s ahorse api ece  now w ith the Ro ck Isl and a nd  Da v e nport o nthe  a nte-season b ase ballga mes.  Rock I sland won
  yester d ayafternoon's e xhi biti o n  g ame  a t T we l fth s treet park 4 to  3, the sa me  score by w h ich  D avenpor t defea ted
the loc al aggreg ation onth e gr o u nds 
  acrossthe rive r a weekp revio u s. 

The  Wo rd Det ec tiv e, af ter co n fir mingthe ph ra se isn 't "i t' s a hors e of peas", said in 200 0:

"A  hors e a piece" me ans ,  as  you supp osed , "more or  le ssequal " o r  "six  ofo ne , h a lf do zeno f the oth e r. " F i e ld  researcher s f or
 19th_century, as_found in the St._Paul (Minnesota)_Daily_Globe (March_27,_1893):

"What did Emperor_William say to
_ you when you_approached him, Kelly?"
__asked McKenna.
"He didn't say a word until I approached him," answered Kelly. "Then
 _he_told me_to_keep_quiet because the
  president_of France was listening to
_ our_conversation. I didn't care a d—m
  for_the_president of France,_so I said to
  William: 'He can't hurt_me. Let him
  listen. It's_a horse apiece,_for_I'm_the
  King of_China.' When I saw the Prince
_ of Wales I asked him_for a chew of tobacco, and kicked_because it wasn't the
  brand_I was used to chewing._He tried
_ to get back at_me by saying_no man_of
  my_nationality and name could talk
 _back to a_peer of the realm. I said_to
_ him: 'Come_off_de_perch, you_want
  do_see?'

For_some background,_the_article says 'Workhouse' Kelly was a_nearly_60-year-old Irishman who had lived in St._Paul for the previous_20_years, spending 90 days_to six months of each_year in the workhouse. He had_been a_sailor in_the US navy in the 1850s and was "full of yarns".
The_other two uses I found in_Chronicling America are both_from the_Rock_Island (Illinois) Argus._First_from May_31, 1899 describing two baseball teams as_appearing equal_before a match:

It's a horse apiece._The shake-off occurs today.

Second_from_April 18, 1904, also describing two_baseball teams:

It's a horse apiece now_with the Rock Island and_Davenport_on_the ante-season baseball games. Rock_Island won
  yesterday afternoon's exhibition_game
  at_Twelfth street park 4 to 3, the_same
_ score by which Davenport defeated
__the local aggregation on the grounds
__across_the river a week previous.

The_Word Detective, after confirming the phrase_isn't "it's a horse of peas", said in 2000:

"A_horse apiece" means,_as you supposed, "more or_less_equal"_or "six of one, half dozen of the other." Field_researchers for