These are PCB holders - two pieces of wooden dowel about 2 inches long, connected by some threaded inserts and a bit of threaded rod. Screw them together to clamp them onto the edge of a PCB and it stays nicely put, above the bench, for easy soldering. Assuming there's open space at the edges of the board somewhere, it lets you flip the board over easily to switch from soldering to debugging, and keeps any tall components from banging on the bench when you do so.
When I'm working on a System 11 board, I can even leave clip leads for power connected when I flip the board, solder something, and flip back.
I was looking for a cheap PCB holder, and couldn't find anything that met my definition of cheap, but looked like it would work for large PCBs. I did however find a reference to a test fixture system that used heavy metal cylinders that screwed down onto the PCB. I realized that if I did the same thing in wood, I'd have the perfect tool for what I'm doing, and one that fit my budget.
My first attempt (shown in the pictures) had some issues with cutting the dowel flat (my manual miter box has more slop than I realized) and with drilling the holes centered (my drill press is all over the place). My second run went much better.
I don't think the whole mess cost me more than $15 for six of those, which was plenty to clamp onto a System 11 board for bench work.
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