[Milsurplus] DAG Receiver: What Is This Thing?

[David Stinson]

This is the top of my DAG receiver:
https://goo.gl/photos/55fu8uG5MFuzRe3AA

There's a threaded cap on the right side that covers
the sense antenna port.
The one of the left, for the loop connection, is missing.

My question:
If I were going to ask some outfit like McMaster Carr
about one, what do I call it?  How do I specify its
size and thread?  What do you call the ball/chain 
keeper that is screwed to the chassis?

Thanks, 
Dave S.