I almost responded, but the info in my mind was for a emisar light.

I’ll be checking the s21 in a minute here at work

Edit: ok… .743 od on the brass ring. Kind close to 19mm if you ask me.

Also with my old eyes, it appears to be about .75mm pitch

And it’s left handed as well.

Start with that, 19mm x .75 pitch, LH.

I don’t know what’s common for metric, as .7mm pitch is also close