Why would there be a fake P-mag?. It would not work for any airsoft stuff
Generally PMAG's are stamped with PMAG logo, but might be on the other side. Hard to say with the various versions they have.
Just guessing, but the OE could mean "Original Equipment" which is included in various factory guns that have Magpul mags and/or furniture on them. The big blank spot at the bottom looks the same size as the mags with manufacturers logos on them:
Ah yep that's it right there (without the M&P of course). Brownell's must have gotten some leftover blanks or something. Thanks
Excellent detective work. Yes the mag you have is from the OEM production molds with no branding. It is identical to the MRev magazine apart from the rib depth and spacing.