I usually call it semi rigid cable.
I was thinking the same thing. Possibly the boards at the connectors are to give a good ground at the center pin. That is, with a ground plane on the back side. The semi rigid outer conductor may be soldered nicely all around to ground on the back side, and the center conductor coming up through a hole to the front, maintaining a nice 50 ohms.
It would be interesting to see what is going on with the other boards and why the complicated little board with the two perpendicular pieces.
If its a hybrid, zero degrees or 180 degrees or 90 degrees, the bandwidth defining spec would probably be isolation between the two ports. Terminate the common port, then put a signal in one port and see what comes out the other. I'm wondering if the frequency band is actually higher than you have looked.