From what I have been reading in our database that is being developed, for the years you are talking about (between 1990 and 1994) , it is pretty simple:
First series of numbers, either 3 or 4 digits, this is the frame number, in the order of production
Next character is the frame type, i.e. "Y" is Yo Eddy, "A" is 10th Anniversary, etc.
The following character is the last number of the year in the 90s, meaning a "2" would be 1992 and a "0" would be 1990.
The last characters are the sizes, like "M" for medium or "ML". Some bikes that used measurements also use the last digit in the measurement, like an 18" Wicked would have an "8" instead of a letter.
This all changes after the move in 1995, though, where the frame type gets put first, then size, then the frame production number. Pre-1989, though, seems to be proving to be a bit tougher to decipher, as the pattern jumps around a bit more.
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