Shortgrub,
You can get a pretty good approximation of the groove diameter from the chamber cast, or a lead slug driven into the barrel, with a digital caliper by lightly closing it on the slug, then "rolling" the slug until it "clears" and check that reading. You should do it a few times until there is a consistent reading. A friend of mine wraps the slug with a piece of his thinnest shim stock and mikes across the diameter. He then subtracts twice the shim stock thickness for another way of approximating the diameter. If you had access to pin gages, they may have also had ring gages to match.
Mike
