More on this; I shot my carbine for the first time yesterday, burning though about 50 rounds of S&B 110 grain ammo. Of the three magazines the gun came with, two were complete jam-o-matics and only one was totally reliable. The bad mags had the exact same kind of malfunctions, the bolt would override the base of the top cartridge, jamming the round against the base of the feedramp and the bottom front edge of the bolt.
None of these mags have any type of markings or stamps on them, and I have no idea of the manufacturer. The only differences I noticed between them have to do with the finish and how they fit in the mag well.
The good one seems to fit a lot more snugly. There's a rib that runs down the front of the magazine and on the good mag this rib is wider and more square in profile than the other two. On the problem mags this rib is narrower and more rounded in profile and doesn't seem to fit quite right, causing some slop. The finish is also different. The problem mags are blued and appear to be recent manufacture. The good mag has what looks like a sprayed on black finish, not blued and not parked, and also appears to be new manufacture and not surplus.I have four of the Korean mags on order from Aim Surplus and I'm hoping they fit and function better.