Here you go . . .
Unfortunately this is the best picture I have for now - but there is also a protrusion on the top of the barrel at the back of the threaded section that has what appears to be an index mark on its center. You can see the edge of this protrusion in this picture, but nothing more. The more I think about it, the more I am interested to learn the origin of this interesting machining. Whoever did it, it took some time and attention to detail.