A big 2 5/16" (58mm) hole, and four small #18 (0.1695, just a shade
smaller than 11/64") screw holes with centers spaced 2 5/8" (67mm) apart
(ie, the little hole centers are 1 5/16" from the center of the big
hole). For comparison, the equivalent numbers on the next diagram for 3
1/8 instruments are 3 1/8" and 3 1/2" respectively.


Thanks for the quick reply!

When you say #18 screw holes I ssume you mean #18 drill. Are the screw
threads 6-32?
4-40?


PS-
Not trying to talk down to you in my description either

Not at all. I appreciate the clarity.