Those look just like the same 50 round mags that I bought from USMG a while back (for about $90 for a 3 pack).
One curious thing I noticed of those are two of the mags didn't have a reinforcing bar on the top front at the seam and they were just spot/tack welded, while the other mag did have one.
The followers in the USMG ones are made of plastic. And I have yet to be able to remove the floorplate on any of them since the lips look like they've been bent inward for some reason and I haven't pried on them to get them apart.
I haven't tried them yet, so I don't know how they'll run, or if loading them to capacity will cause them to swell out, but the seam of one of them looks pretty iffy since I can see light through it without any rounds in the mag.