Sindone, thanks for the photos!
I think it very well could be a cast receiver, although I'll state that I'm no expert. What I find interesting are your 7th photo (apparent "pittiness" and rough, "cast-looking", un-machined corner of the receiver), your 10th photo (where it seems that the bottom of the receiver has been machined, similar to the one I posted recently; also, we can see the "cast-looking" corner of the receiver), and your second and third photos, which include some interesting details, to wit:
1. The barrel has an index mark, with no apparent matching index mark on the receiver (somebody check my ancient eyes!!);
2. The receiver has the "flats" machined in to the sides of the forward-most part of the receiver; and
3. The rough edge (indicative of casting?) between the left side and the bottom of the receiver.
Which of your photos would best illustrate the faint casting line?
You've also provided photos of the receiver cover, which appears to be "pitty".
This is pure guessing / conjecture: could the PLA have refined their casting process to the degree that pinning was no longer necessary, such that a threaded barrel / threaded CAST receiver were a viable option?