2 points
A couple of more pics.
3 points
I think I found the screw plug.
https://www.phoenixcontact.com/en-us/products/plug-for-cable-screw-gland-hc-bs-pg16-1674516
3 points
Looks like a spanner nut and the cap appears to be a jam screw to lock it in place.
2 points
Need to see the bottoms of those pieces.
12 points
The indentions on the side are for a spanner wrench to tighten this nut onto a threaded rod. The top is a decorative cover.
2 points
*
The hole looks like it’s for a set screw to me, but I agree the cap is just a decorative cover. Not sure what this would be used for though as I don’t understand what function a collar and cap like this would do mechanically.
Edit: I see OP mentioned the holes don’t go through so you’re right they’d be for some special tool.
