The blue print uses B, the center of the selector to the center of the sear pin hole. The A reference measurement I’ve seen online varies 0.022”.
This is why jigs, like Bowers, are popular. It’s tough to accurately measure a hole. If you eyeball the center with calipers, there will be a certain error & the reproducibility of it will likely be inconsistent. The tolerances then become the issue: how many thousandths off can you be & still have reliable function?
If you use a jig, you use proper sized pins to center it: 0.125” for the top hole & 0.375” for the selector, then the jig either fits, or it doesn’t. Jigs are the “Go/No Go” gauges of the machining world, or they were when I ran a mill.
The only accurate way I know of to measure center to center of holes is on a vertical edge mill with a center, which is a tapered point that finds the center of the hole from the center of the chuck.
I’m sure a real machinist will come along with better info, but the older Bridgeports machines with X,Y,Z axis you crank by hand measured with a center pin and hand crank while watching the axis measurement. CNC—I know nothing, but have seen laser measuring on TV’s “How it’s Made.”