Actually, it could be interesting to try to make some of these bushings on my CNC lathe. I have no brass, but I have aluminum and cold rolled steel in a size that could make some. Hmmm...While working on the cylinder for my Webster Engine, I started with a piece of steel (cold rolled steel - CRS) longer than I needed, so that I could be sure that I had plenty to hold in the lathe chuck's jaws. When I cut off the excess, I noticed that the brass collar for the router bushing almost would screw onto it. The cutoff steel was small in diameter by just under 1/32", perhaps the depth of the threads (?) but it has been staring at me for days, daring me to try to thread it, saying, "what do you make of me?" and I finally broke down to try.
The piece of steel I cut off is 1.148" diameter, while the bushing that I measured across the tops of the threads (major diameter) was 1.175, or .027" bigger. I went back to the CNC threading lathe setup I got working last winter and had a reasonable guess at the thread parameters ready within a few minutes. Ran the code and no one would be more surprised than I am that the collar threaded on first try.
The collar (brass) works, but the issue is that the tops of the threads don't have the expected, 60 degree, upside down Vee profile they should. Instead, they're flat topped from not being cut deep enough. I backed off the collar a few turns so you can see it.
This piece is all the wrong size and shape, it's really just a silly experiment, but it's not like I don't have dozens of little cutoffs like this piece of steel all over the shop. One with threads is No Big Deal. But I think if I started out with the real part to get some better dimensions of the threading, I could convince the CNC lathe to turn out a few of these.