cheers for the replies.

It is for a trial sight which was used on a No5 .22 trials rifle (Skennertonicon, pg297).

Mine has broken off the head so i only have the threaded part left. This is how i bought it, thought it was a common sight but i have been told the right collector may find the sight more valuable than the No.8 rifle its on.

From what i have measured, my thread appears to be 1/8" and about 40 TPI, single pitch. I was thinking of trying to get a jungle carbine elevation screw so i can turn and cut the required thread on the section that isn't threaded. This will allow me to use the standard click section and knurl, which appear to be the same.