Someone will eventually ask you to identify your Johnson outboard. If you look about halfway down your Johnson's starboard side, you might see a metal plate with the serial number that contains code letters that identify it. That's great if your Johnson was created post-1980, but if that Johnson you operate was built between 1969 and 1979, the identifier is in the model number -- also stamped on the identification plate -- rather than the serial number.
Numbers, Numbers
Prior to 1969, Johnson model numbers were year-specific; serial numbers were used in identification. Johnsons from the 1969 to 1979 era use the last two digits of the year at the end of the model number to identify the model year; for example, a Johnson model number ending in "-4RL79" was built in 1979. Johnsons built in 1980 and later, however, have two letters from the acronym "INTRODUCES" at the end of their serial number, not the model number. In this simple substitution cipher, the letter "I" equals "1"; "N" equals "2" and so on. The "S" equals zero. A serial number ending in "IN" was built in 2012. A Johnson outboard with a serial number ending in "SS" is from the 2000 model year.