A T2 thread, also written T-2, has a diameter of 42mm and a thread pitch of 0.75mm. It is also sometimes written as M42x0.75.
The low pitch distinguishes it from other M42 threads.
Baader
Quick change system, heavy duty, T2
$ 122.00
ready for shipping in 1-2
weeks
Baader
T2-ring ultra short for Canon EOS
$ 44.90
ready for shipping in 1-2
weeks