Up through 1979 (the latest my list goes) the year of manufacture can be determined from the model number - but you didn't include the model number. It is usually on a small plate attached to the mounting bracket.
1987
The year model for Johnson and Evinrude outboards is determined by the "model" number, rather than the serial number.
your Johnson is a 1985 year model
1986
1971
1985
Johnson outboards, model RDS-24L, would be a 1962 year model, 40 hp. the boat motor is a 50 hp.
Johnson outboards use the model number to determine the year model of their engines.
A 200 hp Johnson outboard, model number J200STLCDR is a 1986 year model.
Johnson outboards uses the engine model number to determine the year model of the engine.
A Johnson outboard, model number CD24D is a 1967 model, 6 hp engine.
The letters in the model number designate the year model, when compared to a model / year guide.
The year is 1992 -the "en" in the model number identifies the year.