The hole and the bolt look like they're for clamping this to the table of a mill or drill press. The semi-circular notch might be for adding a clamp to stop it all spinning round. Then it looks like there's a channel and another clamp for holding the work, perhaps something that already has a 90 degree bend in it as this would explain the small round hole at the back. Maybe it's for holding narrow tubing such as for a rack while notching it? But then I would have expected that channel to have a round bottom. But this is my best guess.