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.

Omegon
Extender 30 mm T- thread
Sale price:
$ 29.90
$ 18.90
ready for shipping in 24
h

Omegon
Adapter M48 male to T-thead ring female
$ 24.90
ready for shipping in 24
h

Omegon
Visual T male to 1.25” female with set screw
$ 34.90
ready for shipping in 24
h

Omegon
Extender 7 mm T-thread
$ 29.90
ready for shipping in 24
h

Omegon
C-mount male to T2 adapter female
$ 24.90
ready for shipping in 24
h

Omegon
Extender 17 mm T-thread
$ 29.90
ready for shipping in 24
h