What is this tool?

[mklotz]

Well, it's way too flimsy to be a nut splitter. That tooth and the swing-away-for-loading design shout (light) chain breaker to me. (Link breaking tool is one of the suggestions.)

It might also be a crimping tool of some sort; several of the guesses thought so. Crimpers, though, generally operate perpendicular to the object being crimped. This thing applies the pressure along the line of the object passed through the opening - exactly as you would want to if link breaking. It could still be a crimper but I would stick with my chain breaker guess if pressed.

The last four guesses...

-Handle on a steam engine's engine brake valve.
-Wire twister.
-Diesel engine spring compression tool.
-Fine adjustment tool for soft copper tubing.

sound completely nonsensical to me.

Searching on the manufacturer, Voss Mfg & Dist Co, didn't produce anything suggestive.

In the days before generalized tools, there were no end of special purpose tools built to do a single job. (Think of all the ways to make a hole in something prior to the invention of the drill press and mill.) Unless some documentation survived, puzzling out their use is going to be tough. Anecdotal evidence is useless - anyone can make up a logical story.