I recently bought a very used but complete Lufkin 680A Inside Micrometer set (for bores from 1.5" to 8" diameter, resolution 0.001"). I don't know how old it is, but the holder has a patent number on it (1,615,488, issued 25 Jan 1927), so it's probably from the 1930s. It looks it.
Anyway, I've been cleaning and adjusting it. Everything was a few thousandths short, largely due to wear at the contact tips (which do not appear to be hardened). The extension rods are easily adjusted with a wrench, but how to adjust the micrometer core (screw and scale) is less clear.
I tried to find a patent number for the micrometer itself, but didn't succeed. That isn't to say that none exists.
Does anybody know anything about this micrometer?
Joe Gwinn