The thread is M46x2 (on the backpate, on the spindle there is a M46x15).

Hre you are the drawing of he spindle end from the technical manual:


It's not very good copy, so if you are not sure about some dimension just ask, I can try to figure it out from my paper copy or measure on the machine. Also please note, that the backplate centers on the big outer taper (8 degrees) and has a driving pin that fits into 13x3,5 slot at the end of the spindle. The pin diameter is 4mm.