There were a lot of anonymous modifications made to surplus firearms back when a Free American could have one delivered to his doorstep at a low price. Pinning them down is tough.

What boggles my mind is "The thin sheathed metal over brass..."
How, why, would anybody make a rib that way. Think of the WORK.
I trust you mean "metal" in the old colloquialism of "metal" = steel. Have you checked it with a magnet?