Re: merc 20hp
[colour=blue]If you are talking specifically about the merc 200(20hp) there are 4 different build locations - U.S.A., Canada, Belgium and Australia. Each country has it's own set of serial numbers.
U.S.A - All begin with either 1, 2, 3 ,4 or 5 (notice that there is no 7).
Canada - All begin with either 1, 2, 3 or 7. 1, 2 and 3 where imported from the U.S., 7 was made in Canada.
Belgium - All begin with 9
Australia - All begin with 8.
You will not find the exact number. Most sites will quote a range. For example, MaxRules quotes the range 7108948 to 7124213 as 1978 to 1979. Your motor falls within this range.
If it was mid eighties the serial number would begin with a letter (that's a whole nuther story!).