I picked up a random piece of RF hardware at a flea market. I'd like to know what these unusual connectors on it are.
The jack is flat on its face except for the gold ring. Inside the gold ring appear to be fingers gripping the central pin (which does not extend out of the connector). The plug's center contact is a helical spring which presses flat against the gold ring.
The diameter of the threads is 17.4 mm outside the jack and 16.2 mm inside the plug (slightly larger than N or UHF connectors). The insulating gap is 11.8 mm and the outer diameter of the gold ring is 7.43 mm.
The plug is actually an adapter to a BNC jack (which does suggest the possibility that these are not standard connectors at all); here's a picture of them fitted together:
The entire unit has two of these connectors, one N connector, and what looks like a mechanically tuned stub for somewhere above 900 MHz. I haven't been able to clearly determine any of its functionality yet.