One more update - the eyepiece end of the rotator is M54x0.75 (learned the hard way) and if you unscrew the small red ring from the larger red ring it exposes an M68x1 thread, which is also a nice bonus. The visual back screws into the M54 thread.
So you have at least 3 threads to play with - M63x1 before the rotator, and the above two threads after the rotator.
TS sells a rotator + visual back separately which seems very similar and has specs which for the moment appear to match what I have been measuring/testing on the AT72EDII and AT115EDT:
I am experimenting with threading 2" reducers/flatteners into an AT72EDII and AT115EDT using the M48x0.75 threads on the front of the 2" nosepieces and (hopefully) using the M48x0.75 threads that some or all of the reducers/flatteners have after you unscrew the nosepiece.
More details (including link to an AT92 sub-thread where a few different people threaded AT2FF's onto their scopes) at the following page.
At the time most of them were using an M63-M48 adapter going in in front of the rotator (which I didn't want to do) but I haven't finished the thread yet.
Edited by bridgman, 08 April 2020 - 01:35 AM.