I found this on the Registry's website, so it looks like you are right (except my plate does not seem to follow the rule. My plate renews in August, but the last digit is 7.)
In 1968, the introduction of the staggered registration instituted a plate that expired every two years with the expiration month determined by the last numeric digit on the registration. Thus, a plate that ended with a "1" expired in January, a plate that ended with a "2" expired in February, etc. Color coded plate decals with expiration years printed on them stemmed from the advent of the staggered system and are still used today.
no subject
In 1968, the introduction of the staggered registration instituted a plate that expired every two years with the expiration month determined by the last numeric digit on the registration. Thus, a plate that ended with a "1" expired in January, a plate that ended with a "2" expired in February, etc. Color coded plate decals with expiration years printed on them stemmed from the advent of the staggered system and are still used today.