Pete, That is my gauge, the stem is 5/8" rod the bushing is a 5/8 x 3/4 brass bushing the collar is a 5/8 shaft collar (hardware item). I turned the taper on the bushing on my lathe but I bet you could do it on a drill press with a file. The brass bushing is free to rotate with the barrels.

I learn something every day, and a lot of times it's that what I learned the day before was wrong