It's an HMG flat and it's literally good for nothing if you want to build a real MP44.
With a little bit more enthusiasm they could've made real MP44 flats and sell them to builders to subsidize their ugly rifle development.
While embarking on their brain fart journey of making the rest of the rifle work with different calibers from .22 short to 88mm flak, they could have ensured considerable incoming cash flow from selling MP44 flats, butt stock ferrules, etc. in an effort to finance their multi-caliber experiments.
You can tell it's an HMG flat by the magazine latch on the wrong side and an MP40 style bolt hold-open cutout.
A receiver flat is a non-regulated part. Seems like it would be okay, but that's all you to sort out.