Surprising that no one has replied as yet here. The Finnish KP-44 (M-44) mag well is part of the upper receiver which is likely why it was cut to BATFE tech specs. The Russian PPS-43 version has the magwell as part of the lower assembly (not receiver).
There are upper KP-44 semi-auto receiver pressings, "80%" types available from which the magwell is reconstructed. The magwell is just a cut-out notch in the sheet metal. It is boxed for rigidity.