I would love to help you with this, but I found the database for Mercury serial numbers at Mariine Engine (dot) com, and their database jumps from 7.5 HP to 50 HP! There is nothing in between!
If it is labeled as a forty horse Merc, it is certainly older, as their database goes back to only 1977 - which (ouch!) was 34 years ago. The only thing I can say for sure is "pre-1977".
A 65 hp Mercury outboard, SN 6315551 would be a 1976 year model.
In addition to the engine serial number, the horsepower rating is needed to determine the year model for a Mercury outboard.
The 20HP Mercury outboard with serial number 9137627 was made in 1979.
The serial number 9137327 for a 20 HP Mercury Outboard is not not listed in the Mercury Marine parts catalog.
You may have mistaken the '1' for a '2'..if your engine is a 4hp 2 cylinder it dates from 1978.
To determine the age of a Mercury outboard, you will need to know the engine serial number, and the horsepower rating.
A 9.9 hp Mercury, serial number OG031959, is a 1995 year model outboard.
Two things are needed to determine the age of a Mercury outboard, the serial number, and the horsepower rating.
A 9.9hp Mercury outboard, serial number OB120743, is a 1987 year model.
An 20 hp Mercury outboard, serial number 3143107, is a 1971 year model.
Two items are needed to determine the age of a Mercury outboard, the serial number and the horsepower rating.
In order to determine the age of a Mercury outboard, in addition to the the serial number, the horsepower is needed.