I am holding my sks in my lap and looking at it disassembled, the magazine well where the fixed mag goes has a latch mechanism in front of the trigger guard,which releases the bottom half of the hinged magazine. The trigger assembly has a fork in the front that holds the top half of the magazine in place when the trigger assembly is snapped into place.The front of it attaches approximately 2 inches in front of the chamber mouth under the chamber. The stock ak mag is not as long in the front. It has no where to attach to. It is rectangular shaped.
The bottom of the chamber of the barral is smooth on the sks.
There is an adapter (I used to have one that I discarded after the ban went into affect, naturally,) that fits into the well where the fixed mag is supposed to be, made of stamped steel, and in IT there is a well that is the correct dimensions to accept the ak mag. The AK mag will not fit into an sks mag well without the adapter. It is not interchangable. I suppose you could JAM it into the hole, but not function well.
At least not on the one I have in my lap, which is a bona fide chicom sks war trophy with matching numbers.
If you indeed have sks's that accept the ak mag, what you most likely have are sks's with the adapter in place, or perhaps one of those "paratrooper sks's " that was the subject of the post I originally responded to. What i was referring to in the previous post was in answer to the postwed pic of the "paratrooper" sks, with ability to accept the ak mag. Perhaps that is what you have. :-))
In either case, they either included an adapter for the sks to accept the mag, or modified the bottom of the chamber metal to accept the front lip of the ak mag, and closed out the hole in the stock
There are 30-round mags designed specifically for the SKS.