Mounting a circuit board atop the Victoreen 710-104 ionization chamber requires figuring out the location of those 6-32 studs:
Given that it dates back to the early Cold War days, the bolt circle dimensions are all hard inch:
I embossed the studs into a pad of Geek Scratch Paper, eyeballed the stud-to-stud spacing from a cheap ruler, back-calculated the BCD, rounded it from 2.742 to the obvious 2.75, then fed that into the first BCD calculator that appeared in the obvious search.
The can is just over 3.5 inch OD and stands 1.5 inch tall.
The can will run at +24 V in relation to the rest of the circuitry, so the studs must be insulated from the PCB’s copper pours. That, most likely, will require some 3D printed doodads.
The circuitry must live inside a grounded metallic can that excludes random electric fields. Somewhere in the pile, I have a few sheets of Mu-metal that, while grossly overqualified for the task (even without heat treatment), should solder up nicely…
The Smell of Molten Projects in the Morning 
Mu-metal excludes magnetic fields more than electric ones. Copper is traditional for shielding electric fields. I’m guessing you want to do both. I had no idea mu-metal is easy to solder. I do know it loses its magnetic shielding properties surprisingly easily when manipulated.
There’s probably some copper foil tucked away in the sheet metal stash; I found the Mu-metal a while ago while I was looking for something else, so it’s the first thing that came to mind. Given that Victoreen didn’t include much more than an aluminum plate shield in the Model 710 meter, I should probably look harder for the copper…
I had a similar issue yesterday, and took a different approach. A new (universal) tractor seat needs a strip to trigger the operator presence (AKA deadman) switch, and there were a couple of handy 3/8 NC nuts in the shell. I screwed a couple of bolts in the nuts, aligned the flats and measured top-flat to top-flat with a caliper. 2.009″ or so, close enough for tractor work. I’ll go slightly oversized and it should work well. Other variants are “left as an exercise for the student”.
You may already know about this but machinists sometimes use tooling buttons of known OD that they screw into holes, so they can calculate centers through OD measurements.
No tooling buttons in stock, but flats are fair substitute for the level of precision required. I drilled 2.00″ on centers with 1/32″ oversize, and it fit nicely. The 2″ was a guess of errors from yesterday’s festivities, and I used a mill-drill to keep things lined up.
Aye! Having never done anything requiring that much precision, I have no buttons. Faked it a few times with ordinary screws that came close enough for my simple needs.
So, as your last dying act, you must fall off the seat… got it!