It looks to me like a flange on the stock of the problem rifle may have been sanded off for some reason. The other rifle still has the wooden flange on the stock where the magazine inserts.
Can you go into a little detail on what section you are talking about here. I just read your post on gunboards and you say when it jams, the round doesn't quite make it far enough up to chamber. It could quite possibly be not installed correctly and this is the cause of your problems.
I guess this is a good example why paper and pen might be a good accessory to take to the range :-) My memory is a bit hazy, but I recall seeing in one case, a round not making it up to the chamber, with the bullet end pressing into the area just below the chamber itself. In another situation, I believe I recall seeing the round attempting to chamber from a canted or slightly diagonal position.
The jams seem to occur only after several successful rounds have already been fired. The last time out, the jams didn't happen until after at least 40 successful rounds were fired. After that, the jamming became frequent. The rifle had been cleaned before use. Typically, I don't load to the 10 round capacity. I'll insert between 3 and 5 rounds or so at a time.
My functional SKS has a cutout into the (inside) side of the wood stock at exactly the place where the magazine inserts. This cutout extends down about a half an inch or more from the top, before ending. Below this point, the wood is thicker, or tighter where the magazine ultimately rests. When I looked at the stock of my problem SKS, there was no cutout visible. Instead, the entire side of the wood stock in the vicinity of where the magazine sits was flush, unlike my functioning rifle. So there is a difference between the stocks of the two rifles. Since there was more sideways space within the problem rifle, I just assumed that someone had at some time or another, sanded off the extra thickness of the wood around where the magazine would be held.