So I have had SKS's along time and many variations, I have Norincos and Yugos and have done machine work myself on several. The question I have is I recently in a trade picked up a 1964 Yugo that was using duckbill magazines and came with a SKS stock fixed magazine and bunch extras. Well I went thru the whole firearm checking dissembling everything to make sure no cracks, rust or anything. I decided on resemble to put the stock 10 fixed back in that was not installed when I got it. Well it doesn't fit. So I take one off another Yugo it doesn't fit, nor does a Norinco off a 72. Now I am puzzled under very very close inspection no grinding was done on the firearm or removals of metal and the duckbills fits nice and snug. The fixed mag that came with it does not match the numbers on the firearm. However my other 1967 10 rd fixed Yugo also doesn't fit either. I have one pristine 1967 all matching numbers and is a Yugo and wont fit . To answer some questions that might be asked. Yes the receiver was checked and the numbers and marking as Yugoslavia, it also has the grenade launcher and that is typical of these models. I have allot of experience working on these firearms. And why I know bolts can vary and such firearm to firearm, the bolt and receiver numbers match so it not relates to that either. To note the duckbill work fine. So why cant a fixed fit ? Open to any ideas I picked it up 4 months ago and it has me stumped.