What is this “XX” 開 “#” “上/平/入/去” “XX” method for describing character pronunciation?
these're codified verse, each character represent a "value" of particular "property", 廣韻 is the origin.

逼   曾開三入職幫


逼   the character looked up

曾   one of sixteen 韻攝

開   開口音 (開 for 開口音, or 合 for 合口音)

三   等第 (一, 二, 三 or 四)

入   entering tone, 聲調 tones (平, 上, 去 or 入)

職   this one is 韻目 (slightly different from 韻母 finals, there're 206 韻目 in 廣韻

幫   it's 聲母 initials

so, back to your question:

“X” + 開


it's [ 通 | 江 | 止 | 遇 | 蟹 | 臻 | 山 | 效 | 果 | 假 | 宕 | 梗 | 曾 | 流 | 深 | 咸 ] + [ 開口音 |合口音 ]

last, it's interesting that the book used traditional chinese script :)

info added.

ok, if you understand cantonese, literary chinese &amp; traditional script, here it's:

韻攝

in the rhythm book 廣韻, there're 206 韻目, which was "assimilated" into 16 韻攝, that, ignoring initials, tones; similar finals were grouped together. i guessed it's easier to remember 16, instead of 206 :)

for example:

通攝, which included "東韻", "董韻", "送韻", "屋韻", "冬韻", "宋韻", "沃韻", "鍾韻", "腫韻", "用韻", "燭韻"; in cantonese (sydney lay scheme), these're:

通   tung1,  sound file

東   dung1,  sound file

董   dung2,  sound file

送   sung3,  sound file

屋   uk1,    sound file

冬   dung1,  sound file

宋   sung3,  sound file

沃   yuk1,   sound file

鍾   jung1,  sound file

腫   jung2,  sound file

用   yung6,  sound file

燭   juk1,   sound file

grep the idea now?

漢語多功能字庫, 鉅宋廣韻

等第

this one, it's roughly louder, loud, quiet, quieter (from 1 to 4). it's derived from 韻圖 (rhyme table).

the internet archive has a copy of 韻鏡, in page 100-101:

https://archive.org/details/02077186.cn



in bottom right, one can find

北
◯
逼
◯


in which, the 等第 of 北 is 一, while 等第 of 逼 is 三