Re: what year is this motor and what should I sell it 4?
The "50th Anniversary Series" motors are 1972 Johnsons for sure. That is the year that Johnson switched from dark green lowers to Anniversary Gold (Lime Gold to us). The model should be something like 50ES72 (50 horse, Electric start, 1972) or 50ESL72 (same thing but long 20" shaft).
I currently see a 1971 Johnson 50 (1st year model) for sale on Kijiji (Toronto) for $450 and it's not sold yet (that should tell you something). All it says is "last time run 2 years ago". Does your motor have the proper remote control? Gas tank?
The 50 is a good motor and was produced through 1975 before a displacement increase brought them to 55 horses. They went on to become 60s (and even later-day 40s with the revised horsepower rating method). The early hydroshift motors are not without their liabilities though.