Originally posted by Ontarian50 I've seen this before. What I think you have there is an M42 lens, but with the inner ring of a t-mount attached.
Someone mistook M42 threads for t-mount - they're close, but not the same.
Anyway, you've got the useless inner ring stuck on there. With enough grip, you should be able to unthread it and reveal the M42 underneath.
The reason it's not a t-mount lens to begin with is that it has an aperture plunger. While there were some early '70s lenses with plungers and t-mounts (think Spiratone and early Sigma) the lens you have there is too new for that, I think.
Thank you so much everyone! Ontarian50 you was right and I quickly got it off now and my m42 adapter works a treat