Hi Peter,
No, I have not used this fixture. It appears that it can accommodate two sizes of DUT. The rings look like dielectric spacers used to create a 50 Ohm line with the hole used for centering the measurement sample. The long screw is spring loaded so as to press the DUT electrodes against the shorted end of the transmission line on one end and the center conductor from the analyzer test port on the other end.
Vladan
