I bought a large lot of gunsmiths tools off of ebay (he had a large collection of Addis chisels and gouges). The lot included several home made machinist's "Go-No GO" gauges. His gauges were made on a metal lathe and were much more modern looking than yours.
Since you also got this from a gunsmith, my best guess is that you have a really old "Go-No Go" gauge. Maybe for placing the gun sights or something. But, don't hold me to that.
My gunsmith lot also introduced me to a delicate spokescraper. I love this simple tool. I have even made several flat bottom and curve bottom scrapers. Great for working with squirrel grained woods (think gun stock).