The 20 indicates that your gun was made in 1980. Marlin did not checker the stocks on the 39a at that time period.
the stock is what you use to hold the weapon by.
the tf11 only comes with a stock or it doesnt. you can not buy an extra stock for the gun.
Any good stock maker.
Theonespy is the best app for tracking. it's very helpful for you.
Mainly in the stock
Estate sale, garage sale, gun shop, pawn shop, gun show, stock shop
you can but it is hard you have to take apart the gun and pull the stock up and close the gun
Try e-gunparts.com, gun shows, gun shops, custom stock makers.
It is the end of the stock as opposed to the end of the barrel end of the gun
it is an additional butt end to the gun that helps you stabalize the gun
Try these sources: Numrich gunpartscorp.com Jack First jackfirstgun.com Bob's Gun Shop gun-parts.com