It is on the upper arm on the back of the receiver.
Do you mean on the upper tang? Some pictures would be helpful. One thing you can check is what the barrel markings say where it indicates the cartridge it is chambered for, does it say Micro-groove rifling? Marlin started using micro-groove in 1956, so that would tell you if it is newer or older than that. However, with no letter code/prefix to the S/N, that means the gun is newer than 1968. You might also just call Marlin.
