English to French Tech/Engineering Mechanics / Mech Engineering

This refers to an adapting piece used to mount the top part of a (modern though retro-looking) four-sided street luminaire on a mast. It must be distinguished from a solid frog, as it looks more ornate and indeed seems to be open in its central part (according to the diagram I am looking at).

I hope this description can help, I will open another query for the "solid frog".