As Jon says, it's 6.5mm from the T thread body connection.
See:
https://astronomy-imaging-camera.com...asi1600mm-cool
The mating thread on the corrector is M56 x 0.75 - Ideally a 50mm spacer with female M56 on one end and a female M42 T thread at the other.
Don't know where you'll get a "special" like that......