My brother showed up yesterday and gave me this tool, asking if I what it was. I knew what it was for almost immediately, as there could only be one use for it. It is from the estate of a local custom knife maker here in Idaho, but that's not the original trade it was made for...
It is a thickness caliper made by a company by the name of Voit & Geiger (Chicago, U.S.A.), one of a fairly simple yet ingenious design, used for gauging the thickness of a violin top (or bottom) when carving it out. Being curved, the thickness of these pieces cannot be measured directly without such a tool - this allows you to see the exact thickness at the point the caliper is set to.
I've seen hand-made versions of these, made with a dial caliper and usually a plywood body. A modern manufactured version is available from Stewart McDonald:
This tools does not use a dial caliper, rather it uses a rather ingenious lever system to show the thickness of the wood at the given point. The lever on the back opens the caliper and is connected around the half-way point to the front arm that displays the thickness in fractions of an in and in millimeters.
Here you can see it opened up, and the relationship of the two levers:
Here's a shot of the back side:
Voit and Geiger, Inc. were violin makers in the Chicago are from 1925 to sometime in the 60's or 70's. There's a bit of history and an example of their work here:
They also aspired to sell violin-making supplies to hobbiest, as shown in this ad from a 1938 Popular Mechanics magazine:
This is likely one of the tools they sold through that catalog.
Leroy Geiger (the Geiger in the name) also wrote a book to that end,"How to Make Your Own Violin" which I have placed an order for (and will review here if it is worth it's salt). It should be an interesting read, in any case, as it's from the hand-tool era, and from what I can gather, the pair were well respected as luthiers.