What's the thread pitch - 20x1.0?
To be honest IDK for sure.....the pitch is 1.0 but the diameters are screwy....certain parts fit between the 19 and 20 but others don't....the treading is all over the place and most threads are squared off and fairly incomplete.
I'm pretty sure this particular location is supposed to be 21.5mm x 1.0, but I would get one to test fit and measure.
I have a lathe so I can accommodate some weirdness.
The first ones I made out of aluminum and I matched the thread diameter of the original top cap. Then I destroyed my 19 tele tubes in the lathe trying to get cute with a chamfer and caught a tool. So I sent it to d9mel and it didn't last long.
I salvaged a portion of my 19 tube to test fit the new adapter and it ended up at 21.4 OD.