Probably the oil-rotted buttstock combined with factory having installed short screw at front of receiver for bottom metal which pulled the threads. Recoil forces transferred to upper tang only.
I already have a couple of days worth of work into this rifle fixing the bottom metal, bending and filing the lever so it would cloae all the way, getting the trigger block to work again, stretching the sear, replacing screws, repairing threads, and tons of filing, sanding, and polishing of the inside of the receiver and internal parts. The original forearm required a vise for installation before I spent a few hours filing and sanding on it. All that just so I could shoot it.
This rifle was a total basket-case from the beginning. I bought it as a package deal from a CB member with 6x Weaver, Lyman rings, spare poplar replacement buttstock, BRP mould, and at least some of the problems disclosed. After I bed the buttstock to the tang, all of the mechanical issues should finally be sorted out.
Several people have told me how fine and wonderful their early 1950s Marlins are and all I can say is this one was built on a Friday afternoon by apprentices out of leftover parts they found in the defect bin or on the factory floor and was bought by a retarded goat farmer in west Texas who only had a wornout Stanley flathead screwdriver, rusty claw hammer, and a pipe wrench for tools.