the E prefix dates your marlin model 336 to 1948. the a j letters could be someones initials but have no meaning to the age of the rifle.
Gunpartscorp.com
The Marlin model 24 shotgun was produced by Marlin from 1908-1915.
Marlin Firearms
try www.e-gunparts.com
Try e-gunparts.com
Try e-gunparts.com and Marlin
tolerances have been degraded between the firing pin fulcrum and the release. examine pivot-point wear and never exclude the small chance that a single grain of dead powder is between the firing pin and the hammer. reproduction of the original hammer-pin isn't rocket science.....make a template of the original, then add a couple of tenths to the new part.....very nice rifle, and an easy gun to repair!
If your pump 12 ga. has an exposed hammer, it's most likely a model 1898, as all later models of exposed hammer Marlin shotguns had the model marked on the upper tang behind the trigger. If it is not an exposed hammer, then the model designation is on the bottom, where the receiver meets the mag tube.
I have a mod. 336 30/30 that confirmed serial through Marlin Firearms. The serial located on tang directly under hammer.
Yes, at least my '71' model does.
Your rifle is a Marlin 25M. You can check with Marlin Customer Service, expect price to be AROUND $25 plus shipping.
Use a Marlin 57 firing pin. It's the same rifle, just re-branded for Sears.