Ever wonder what's inside a General Radio 1602-B UHF Admittance Meter? I've split it half so you can see the two sides. The body has the slots. The front carries three brass tubes with insulating washers in the centers. The center posts all connect to the detector connector on the front. The other side, shown here, has a wire connecting the center posts to the brass body. It's a one-turn loop. When you change the position of the levers, it rotates the wire loop to be parallel, perpendicular or at some angle to the slots. That determines the coupling. I don't remember how to operate it but you need a bolometer detector, a generator, a variable cap and some DUT. Man, this high frequency stuff is all just plumbing!
When I put it back together, I'll get a shot of the front, or just Google 1602-B UHF Admittance meter. (edit- added)