I don't know who did this one, but that's exactly how I do a joint when stretching a barrel.
On a lathe, indicate on the bores of each barrel piece, cut them to size, thread the two pieces, screw them together and weld the join at the surface.
The weld and joint can be invisible at the surface and in the interior, but the x-ray shows all!
I think the 1 and 0 are just numbers on the x-ray film, and nothing to do with the barrel itself.
John