Likely nationality of both the magazine & then the rifle.
Iv seen some that like Chines norinco's & some that like Yugos. Switch them on the other & you'll be lucky if you can even get it to fit on, Let alone feed reliably.
(can I make the assumption you have a Norinco :3)
As for brands of mags to recommend I haven't picked up any mags of drums my Yugo didn't take well But saying that Iv stayed away from anything that looks shady at the shows.
10 round detach works fine, gives the SKS a FN49 look, But oddly sticks out much more the the fixed 10rd mag But having the same capacity. IDK
Just keep in mind when you take it apart, Don't force it back together, It it doesn't snap back into place check the position of you Safety switch, and try. Should be able to put the stock back on by hand.
Bent my trigger assembly after switch was in the wrong spot and I pressed enough on it. XC
Thank you , I do them from experience Some stuff you can never find the perfect photo of to help, So I know how it sucks to look & look for a single photo, Im hoping I could add a few more of these and make someone's life easier.
As well as to better educate others who normally wouldn't come across this info. Wish some one told me all I listed here before I bought my first SKS rifle, Would still bought it But would have been nice to know exactly what I was getting into.
Hopes it helps Need any refs like this just ask, If im already going to do one, Ill do it first