Simple to make too. slide, reference face, lock.
I made one after seeing what my guru was using (he had made). Got the hole wonky in mine, and the slide is a bit big, so it's not used much. Lately, I've been using what a well-known maker uses at his bench in some video productions--the brass extension of a folding ruler. I'd long had a folding rule, but had only used the extension for inside measurements. Now I keep one on the bench.
Having a scale on the gauge isn't always necessary but can be handy from time to time.
Dave makes great stuff, I'll have to remember to get one of those the next time some money flows by me.