It's none of the above Ken. They out did themselves with this one. None of the threads on the optic window or the camera body are any know standard thread size. I initially bought a c threaded nose piece, that was a waste of $30, I should have measured it better.
There is also an adapter to replace the optic window which accepts C threaded lenses, and they sell the lenses. Tiny complete guide cam maybe or perhaps an all sky camera. Or an Sti wanna be
Still, I have the mono in an OAG and so far have always had a star to guide on.