Marlin didn't start the "formula of subtraction" until 1973 model year and later.
Please look in the "REFERENCE MATERIAL FORUM" here on the Board and scroll down about 15 threads to the topic posted by HOGSHIP about "How to date a Marlin". It is informative for all of us.
Trust me, I am a total novice when it comes to supplying expert info as such but I did remember the original thread and that is why I copied it and sent it in a PM to Cowboykell. It really is good reference material but in your case, hopefully some of the real experts here can steer you in the right direction based on your serial number above.
"Hogger" , Marlinman93, and several others on this board have forgotten MORE than I will ever learn about Marlins. But it sure is fun trying to learn from their sharing of information.
I'm wondering if this serial number might be lightly marked, and doesn't show up well? If you remove the buttstock, the serial number is also on the left side of the upper tang. You can double check, to see if it matches the one on the bottom.
marlinman: imagine my surprise. the real serial number appears to begin with "AB", not "48". the serial number under the lever was worn down enough to obscure the first two numbers. thanks for the tip.