Well Rick, you were close when you said that it looks like someone is missing a wheel hub, when talking about the mounting base for the tail gun. It truly isn't what it appears to be, although it's not a wheel hub.
When I first saw that I needed something substantial to mount the gun to, I could only figure that the factory would've used a casting of some sort and that I would never have the time to manufacture one myself. Now, I did manufacture the female portion of the gun mount (that the yoke actually sits in) out of some nylon/teflon/hard plastic stuff, but still needed the mounting base for it to attach to. So, with that thought in mind, I went on a hunt for something that would work. I didn't really know what I was looking for, but knew I'd know what it was when I found it. Make sense?
Anyway, when I came across this jewel, sitting out on the hill, rotting away, I instantly knew that it would not only work for what I had in mind, but would give the appearance of looking like it belonged there. It's a clutch assembly off of a big, honkin' starter for a big, honkin' engine of some sort.

........
I started disassembling it right away. I'll say this, I have a whole new respect for the folks that build and overhaul the ol' starters for these airplanes. I had no idea of all of the things they cram into these little castings. Heck, at one point, I lifted one collar off and hundreds of little ball bearings just fell out all over the floor! You'll see all of the pieces (that I could find) in the last picture of this sequence. I'm just glad I don't have to put this back together............
Once it was all disassembled, I started turning it into a gun mount. I cut a hole in the top of the old cover so that it would accept the yoke receptacle I built........
Then I hacked off all of the "protrusions" from the bottom side of the main casting. That was a booger-bear, but still took far less time than fabricating something from scratch. I had also started cleaning up and painting other parts of the casting at this time........
And with everything in place (with the exception of some attaching hardware), here's what the final result looks like......
And, once again, in position in the airplane (but don't tell anyone that it's really not what it looks like

).........
Oh, and yes, the tailgunner sits "comfortably" in a canvas sling that hangs at the back of the airplane. I have it being manufactured right now and will hopefully be completed this week. You can actually see it in the drawing in my last post. I have become convinced that this airplane was responsible for a famous saying derived from W.W. II (you know, like "the whole nine yards" and such). The saying "you've got your ass in a sling" very likely came from the design of this tailgunner's seat. Because once your butt is in there, it's in there. They even had a monorail that went from the waistgunner's position, past the tunnel gunner, and ending at the tailgunner, so that he could be supplied with ammunition during aerial combat. Because, after all....he did have his "ass in sling."

(sorry for the poor quality of picture)...........
Gary