1940s vintage Federal TestMaster 1330P-100 machinist's snap gauge for go or no go part inspection. It has the original finger jointed wood case with velvet lined, fitted interior.
I'm not sure of the exact vintage of this tool, but I saw an ad for it in a 1944 issue of Army Ordnance Journal on Google Books.
This gauge opens up to measure about 1". We measured it as 31/32nds of an inch. The needle moves around the indicator dial when we press on the gauge anvil or the thumb trigger below the gauge.
So we believe it works, but we're not machinists and don't know how to zero the indicator and we're assuming that you'll need to adjust and / or calibrate this.
This is worn and missing some paint and the case is darkened from age and has wear to the velvet.