Yep 'GEN M3' i.e. Gen 3.
PTS did Gen 1 and 2 PMAGs and the EMAG when they had the license from Magpul, but lost said license before the M3 was released, hence the fictional EPM line. There's Gen 3 mag replicas for AEGs out there as you've seen from various airsoft brands under all sorts of funky airsoft names, just gotta click the AEG mag section of a few stores and squint at all the pics.
Beta Project offerings were definitely a budget line alongside the proper PTS mags back in the day, I reckon they might've been STAR/ARES OEM. When PTS had the license their Magpul mags were in £15+/ea (or whatever the early 10s price equivalent would be to an EPM now taking in to account COVIDflation) but were fantastic in both plastic construction and feeding/function, whereas it was something like £30-40 for a pack of 6 Beta mags that look 'the same' but they were a cheaper plastic and were all over the place in terms of whether they actually worked or not.
I got an 40 rnd style Gen M3 purely for pics, think it's ARES.. maybe CYMA. It is, mediocre, but the plastic is ok at least and it'll probably feed a few rounds.