I know that tool as a face float, Tony.
It's often just made on a straight handle, but that bent file kind was often a metalworking version.
I'm trying to find links to show, but they're a bit hit or miss. Benchhand joiners, carpenters and the like used them to get into awkward slots and tight spaces. Mortices and the like, or musical instrument makers.
Sometimes called a cranked rasp if it's really coarse, iirc.
It's the kind of tool that only someone who works with wood daily would buy. It's a good thing, very useful in just the right situation, but most folks wouldn't think to buy one or have need of it.
M