What is this small hand held cutter?
Okay, I had a hunch that this was a leather-working tool, but could only find a similar tool for twine cutting, until I got a nudge in the right direction.

This is called a strand cutter, or lace cutter, or even just strander. This particular version is a vintage/antique made out of what appears to be cast iron.

To use this, you would adjust the width of the lace you want using the knob at the top. Then, you put the flat of the adjustable piece flush against your piece of leather and pull towards you. As long as the blade is sharp, and you keep the tool flush, you'll get even strands of leather.

Here's another such tool:



And here's a modern alternative, called he Australian Strander:



The modern tool performs the exact same function, except it's made from chrome-plated steel and takes replaceable carbon steel blades, so you don't have to sharpen it. Tandy Leather has a demonstration video here.