T2 Mounts are made up of an inner ring with an internal M42 x 0.75 thread, and an external ring which is the camera fitting. The two are held together by 3 grub screws, slackening these screws allows orientation of the T accessory relevant to the camera. They are commonly used to fit DSLR cameras to microscope and telescope adaptors, slide copying units, mirror lenses, etc..
Nikon T2 mount
This is an adaptor which simply allows you to fit telescope style T2 lenses to your NIKON film or digital camera. It should be compatible with any camera that accepts nikon lens mount, including DSLRs, so long as they have a “manual” mode, since T2 lenses don’t have any automation inside them.