I cant help with the detachable mag issue; but I will ask WHY??? The SKS was designed as a real go to war rifle with the standard 10 shot mag loaded by stripper clips. It was and is a perfectly good weapon, one that I would happily carry right now in combat. The Chinese experimented with a detachable mag version to take AK mags as a transition then finally switched to the AK.
If you ask me the after-market detachable mags are a sad joke played on the American public to turn a decent war rifle into a "downtown tart", so the uninformed could play cowboy at the local range. Ive carried a SKS in ugly places where people didnt like me, and it woks just fine as it was intended.
Try this practice with the issued version and stripper clips versus the after-market mags, see who can keep their version running longer without fumbles and jams. The strippers are cheap to buy but are built to last. The after-market mags are usually expensive to buy and cheaply made. If you really must have a "Hi-cap" look for the Chinese military 20 round replacement (but fixed) mag.