I'm hoping to find a ready-drilled lens board (lots of them on evilBay right now), but will probably have to go to the machinist. Looks to be 43-44mm using the ruler. Lens is mounted in a Flash Supermatic (with ASA plug!). I don't suppose there is a chart somewhere on line?

Also, anyone know which rangefinder cam this uses? Looks like it could be #5, 6, 7, 31 or 32.

If you have a flange (fat retaining ring with wood screw holes) then I'll bet 444-45mm hole would be fine.

A retaining ring needs to seat in the hole rather than the threads, so Kodak recommends 47mm

The cam will be tricky. Each lens is a bit off from it's stated focal length and Graflex insisted that you had to measure the lens's actual focal length to match it with a cam.