I have a hard time to confirm the specifications of the thread. Originally, I tend to think it's M77 since the objective lens cell has M77, but now I believe that it's not:
First of all, I think it's more like 3 inch rather than 76mm or 77mm:-
The hardest part is the thread pitch, if the diameter is 3", the thread pitch should also be in inch?
And here is how it looks when measured against centimeter:
This is how it looks like when unthreaded, basically, all you need to do it to slide the dovetail outward and you can see a screw directly under the dovetail, loosen it and you can unthread the rear assembly very easily.
Two more shots: