Originally posted by stevebrot What is the brand and model of the lens? There are several users on this forum that have deep knowledge of this sort of thing. A photo might be helpful as well. As Lowell suggested, it sounds like your lens is designed to be used with adapters for different mounts. From what you describe, it is not T-mount. Those have a 42mm x 0.75mm thread. (not the same as M42 = 42mm x 1mm thread)
Steve
steve, what I meant is that if you look at a t-mount to pentax screw mount, the interface is actually a groved ring, with threads on one side that thread into the ring.
I have a miranda T mount lens, and 2 different lenses with M42 t-mound adaptors.
If the OP gets a t-mount to M42 adaptor he should be able to make a ring, to attach the adaptor to the tmount to M42 adaptor.