I just bought a 90 hp merc. ob. I don't have the SN in front of me, but it is 'A17***X' I know that the first three digits are correct. The X's represent numbers that I can't remember. It is an inline 6 cyl. According to a manual that I looked at, the 1986 model numbers start at 'A9***XX' Did the 9 start over at 1 when it got to 'A999999'? That's the only logical conclusion I have come to.