I'm w/ Sniper47 on this one, Plum.
I hadn't known 50-round magazines were available for these rifles ... wow!
If you had just acquired a select-fire rifle (an assault rifle fitted w/ a selector - or "fun" - switch) that is one thing, but you didn't, and that is something else. Keep the fixed ten, like the comrades suggest, and either sell the high cap magazine, or - better yet - run it over w/ your pickup truck so no body gets hurt.
Sounds to me like you are now proud owner of a ChiCom Type 56 Carbine. Cool. Keep it stock; keep its value high and appreciating over the years.
Your rifle is late production. And this is a safety concern for me should you follow advice proffered here and disassemble your bolt assembly. You see, its firing pin may be of the second (or late) pattern too. If you have it out, take a good look. See it has two fins about one third way down the shaft from the tip, one top, one bottom. When re-inserting it back into the bolt be sure the bottom fin is to the rear, the upper fin to the front. Should you fail, you will have a runaway gun shooting full auto all day long. If you're unprepared for that you could well wind up dead.
A safer, easier approach is to take a spare ziplock sandwich bag, fill same w/ paint thinner, and toss your complete bolt assembly therein. Let her sit over night. Squish when bored. Retrieve in the morning. Rattle the group back and forth. See the firing pin move forward and to the rear effortless. Job done.
Much has been made, Plum, about dried cosmo in firing pin channels. In my experience this is no big deal. Did have one stuck firing pin, tho, but that was in a rifle that had been "test fired" by a previous owner and put back in the stable wet.
There are on-line resources, Plum, on field stripping your rifle. And they are good. I'd suggest you procure a copy of Major James F. Gebhardt's THE OFFICIAL SKS MANUAL (Paladin Press, Boulder, CO, 1997, ISBN 0-87364-908-7) for a fuller understanding of this fine weapon and how to maximize its effectiveness real world, real time. Good work. Thank the old **** who sold you the gun for his kindness, for passing along a piece of history to someone who will respect it, for passing along a life saver for the times of trial ahead. Do right by him and do honor to him.