What Is This Tool Used For?

It wasn't just for tests of authenticity. Several stamps in history have had many revisions. While the picture remains unchanged the perforation may. This difference in perfs will allow the collector to differentiate between issues. I have the complete Scotts Stamp Catalogue on my laptop which describes in detail the identification of different perf methods, when they were used and for each stamp it will list all the different perf types the stamp was produced with. Each type will have a different corresponding value. If I remember correctly the 2c King Georve IV red Canadian postage stamp had a 13 1/2 perf variant that has a book value of $50 whereas the more common perf variant is worth 15c.

Heh, for a minute there I thought it was some kind of wet film thickness gauge. You can guess which industry I work in.

It's a postage stamp perforation gauge ?
Made in USA by WHITE ACE ?
(just a guess..)

Looks a lot like my knit stitch calculator. Which includes holes for measuring the size of knitting needles.

It is a device for measuring the pitch of the perforations on postage stamps. Used by collectors in the process of determining if a particular stamp is genuine. This along with coloration, size and watermarks assisted the collector in his quest for the real thing.