I added another photo to this set in which it shows more writing on this tool, "Go .350" on one end and "No Go .370" on the other end. I'm not a machinist so I'm not sure if which idea this bolsters, whether it's a lapping plate or a gage for screens.
The handle part of it is for plug gages but the tool has been modified. It looks to be a lap. It is likely not a commercial tool, made up by a toolmaker for his own use. The gage handle was probably handy at the time.
You might be correct, I don't know enough about plug valves to verify your idea, although the width of the pivoting piece is just barely less than .375 inches wide which is right at the measurements on the handle. Did they make combination lapping tools and go/no-go gages?
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