64
Metal, about 1 inch in diameter
(i.imgur.com)
Let us help you identify that mysterious object you’ve found.
Currently in CHALLENGE mode: If you've got something obscure knocking about, post a picture, and let's see how we do. Please prefix such posts with "CHALLENGE:" so we know we've got a fighting chance.
I think I found the screw plug.
https://www.phoenixcontact.com/en-us/products/plug-for-cable-screw-gland-hc-bs-pg16-1674516