Thx, but I'll admit to being too lazy to do that today, hopefully it's resolved soon. I dropped my phone so it's not working too well with its cracked screen, and it's a simple thing, 2 bearings for the file to ride on as the fixture straddles the piece. The bearings are set for the height of the finished piece, and then an indexing technique is used for the various angles, usually a simple pin locking a gear in place. I would likely just do some bolts threaded into a plate (think faceplate or drill press table) since it likely wouldn't be for a lathe, but rather simply taking something down a few thousandths. One could conceivably lower and flatten small engine components in this way, or use it to "machine" (shape might be a better word) small parts from scrap, which would be a pretty cool and useful trick to pull off with minimal investment and tools. Since the file rides on the bearings you have precise control over the depth of cut and how straight and flat the surface is, basically cheating the skill part of accurately filing something by removing the human part of the equation. It's the same idea as a roller guide for sharpening chainsaw chain, which of course weren't even invented yet when these were common.
Here's clickspring again with filing tips, he goes into other types of guides towards the end.
And here is a video of an updated version of the rest, much easier to understand than a 100 year old mechanical drawing.