Not sure what you have there.
That serial would make it a 1984, Canadian built inline 6 cylinder - so unless you lost 3 cylinders somewhere along the line, that's not a valid serial number for what you have
The 3 cylinder models started production in 1985 and are still being produced today - so that gives them a pretty wide range of "what it might be".
Since it's a Mariner (just a Merc with different paint/decals in this case) that probably leaves it somewhere between a 1985 and 1996 model (after 1996 they didn't paint up the 90's in Mariner colours).
If you can get a look at the side of the carbs (try the right side from the perspective of looking down their throats), that can give some clues.
If you see the casting number WME 3301-9012 that means it was definately built before 1995.
If you find a different casting number or can find an actual carb model number (which would look like WME 10 etc), post that up then it can be narrowed down to within a couple of years.
The serial should be "alpha numeric" similar to 0A1234567 (first zero may be omitted) through 0G1234567 etc and may be located on a welsh plug (silver disk the size of a quarter, similar to a frost plug on a car) somewhere on the powerhead itself...