Sometimes a picture makes things easier to understand. This is a picture of the Stoney point guage and a case that has been modified to work on the end of the guage.
This is the info from their web site as to the thread specs. etc.
Q: What is the thread size for the OAL Gauge? I am an experienced machinist and would like to modify my own brass. Can you provide the proper drill bit and tap?
A: The thread size is 5/16" x 36. This is a special size drill and tap that we can provide to experienced machinists. Contact us for details.
What I do with mine is to use the case on the end of the guage and then I insert a long 1/4" dowell from the muzzle. This, done carefully and consistently, gives you a better feel for the contact of the bullet and the lands. With a little practice it's easy to hold the guage and case in the chamber, use the same hand to move the bullet up to the lands, against the slight pressure you have on the dowell with your other hand. You will develope a feel that will be very consistent. Do this several times, checking the length each time for an average. Also this way there is never a time when you need to get a bullet out of the lands that won't come out by gravity alone.