Jeb, by now the Marlin Firearms Co. has taken over and all of the guns made now are proof tested for modern loads of that era. Your guns prefix should be FW SXXXXX Also 20 ga. guns will now have 2 3/4" chambers since 1936. I would think it is safe to shoot 1 oz. loads. I would look into the manufacturers listing of their stated velocities and pressures and I would try to stay under 10,000 psi. As far as shooting Magnum loads through it, in my opinion I wouldn't. With quail and chuckers you don't need a heavy load. With early season pheasants at close range, 25 yards, light loads are fine. Late season farther shots you might step it up a little. You didn't state whether wild birds or pen raised. Wild pheasants late in the season you will be challenged. Hope this helps and I'm sure you will get other responses