AJ Parker Iris thread pitch question

[Patrick Chadwick]

M .40X10 fine

??? This is not a thread designation with which I am familiar. M is used for metric threads, with the format diameter in mm - pitch in mm. So M 0.4x10 would be 0.4mm diameter, but 10 mm pitch!!!

I have just measured a fistful of A.J. Parker and Parker-Hale sights and eyepieces. The thread on all of them is 0.225" diameter, 40 t.p.i. A nuisance, because none of my model engineering 40 tpi sets quite fits.

Gehmann eyepieces usually have metric threads (I have a couple). The P-H-compatible ones are rare, so measure carefully before buying!.

How can I be so sure? - Been there, tried it, gave up, and bought P-H eyepieces from Fulton's of Bisley!