How old is your Marlin 22 cal bolt action model 783?

The 783 was produced from about 1971 to the late 1980's.

"Starting in 1973, the year of manufacture can be determined by subtracting the first two digits of the serial number from 100:

Example: SN 2512345 would have been made in 1975 [100 - 25 = 75] and SN 94365295 would have been made in 2006 [ 100 - 94 = 06 (2006) ]."

Prior to 1973 Marlin used a one or two character letter code at the beginning of the serial number.