As others have mentioned, it's a copy of a magpul pmag.
I'm not nerdy enough to know which generation.
However I am nerdy enough to know that pts used to have a license from magpul and their pmags were the best available. There were aeg and tm recoil versions available. Tm recoil versions generally go for £30-50 a mag depending on condition.
Pts now do a non branded, slightly different version of the emag (similar to a pmag but with a window), they do them for aegs and recoils, loom for epm mags. They're pretty similar.
There are generally gbbrs version available as well, depending on the platform.
The beta project pmags used to be sh*t in comparison, the models they sell of it now might be much better.