What do we know about the phonetic distinctions between the 甲類 and 乙類 syllables in 上代特殊仮名遣い?


[上代特殊仮名遣い【じょうだいとくしゅかなづかい】](http://en.wikipedia.org/wiki/J%C5%8Ddai_Tokushu_Kanazukai)
is a Nara-period practice in which two distinct versions of certain syllables
(called 甲類【こうるい】 and 乙類【おつるい】, and denoted by subscript 1 and 2 in Latin
script) were consistently denoted using different manyogana. For example, the
word we now write as 上【かみ】 is /kami1/, which could be rendered with the second
syllable as 美, while 神【かみ】 is /kami2/, which could be rendered with the second
syllable as 尾.

The Wikipedia article states that "the phonetic difference [between the two
types] is unclear", so it looks like this isn't a solved problem. Even so, I
would like to know what we do know about the phonetic differences between the
甲類 and 乙類 syllables (if we know anything at all).

