Quote Originally Posted by Newt_on_Swings View Post
I'm looking into this lens as well and I think it is a 25mm thread. You can take a ruler to it and measure it too.
No. It's no where near 25mm. It's closer to 40-41mm, it fits inside an M42 thread. It also has a relatively coarse thread similar to a 26tpi (looks like an M39x26tpi or M39x1 thread spacing).

No doubt it's an odd imperial thread, but that's just a guess anyway.