Here's a preliminar serial number decodification.
A7A'nnnnn (8-characters) for early 1987
AYYmmnnnnn (10-characters) for rest of 1987 and 1988
MYmmnnnnn (9-character) for 1989
Legend:
A and M in first position is fabricator with A= A-Pro and M= Marvel as suggested elsewhere in this forum and others. A in 1987 maybe Japanese, ex: Araya or another.
A' -A' perhaps is the month where A=Jan, B=Feb, C=March,...,L or M=Dec) typical of Japanese factories in the 80s)
Y- Year
m-month
n-bike number