Remove the trigger assembly. It should only be 3 screws. The serial number is located on the bottom of that plate. (At least it is on my model 99 m1)
Serial numbers were not required until 1968.
Many .22 caliber rifles made before 1968 did not have serial numbers. This would include your Marlin 100.
Being that your Marlin model 31 was made between 1913-1916.These rifles were not required by law to have serial numbers if the manufacturer choose not to put them on the firearm.This was not a law until the year 1969.So your marlin might not have a serial number.
your marlin .22 model 60 was made in 1981 because if you take your first two numbers off your serial number you get 19 subtract 19 from 2000 you get 1981 :)
Serial numbers were not required before 1968. Try looking on the receiver.
the Marlin Model 57M (.22 Magnum) was produced from 1959 to 1969. Since most .22 rimfire rifles had no serial numbers prior to the Gun Control Act of 1968, it would have been produced prior to 1968 if it has no serial number.
marlin 22 model 39A serial number Y6410
Serial numbers were required per a 1968 firearms act. Marlin .22's produced prior to this were not required to have serial numbers stamped on the weapon and marlin decided it was not productive to do so. Although they did stamp some earlier models they discontinued the practice. My Marlin model 56 made in 1960 also has no serial number.
1983. marlin, after 73 dropped alphabetical prefixes and used the first two numbers in the serial number to identify the year. Anything after 73, you subtract 100 from the first two numbers in the serial number. 100-17=83. Any marlin works this way.
Marlin has used both model, and serial numbers since they made their first handguns in 1870. However, this did not always include .22 rifles, as they were not always required to have a serial number, until 1968. Many Marlin .22 rifles from the post WWII to 1968 time were not serial numbered. Model numbers were always used, but some models were not marked with the model number. The model 1881, 1888, and 1889 Marlins were not marked with the model, but they were cataloged as these models.
Yopu wont find a serial number anywhere, because these guns were made before the 1968 Gun Control Act which required .22 rimfire rifles to have a serial number. Prior to that many .22's didn't have serial numbers, although some makers did for their own use. Marlin only did so on lever and pump action .22's.
Early firearms in .22 caliber were not required to have a serial number. Your Marlin 81 falls into this time frame.