[DonVoigt]

I'm with Doubs on this one, someone likely "removed" the receiver threads and "installed" by pressing or ?? a modified revolver barrel(.38 special or .357 = 9mm).

I believe you would best be served by purchasing a barreled receiver(upper) and retiring the one shown.

I believe the "pin" was a set screw, one can see the "dimple" in its remaining face where it was ground down.

Measure the diameter of the shank of the barrel that is showing, it should allow us to determine if there are threads left in the barrel or not. If it measures any larger than 18.0 mm or 0.71", then the threads have been removed or reduced.