To test the detector diodes from wavemeters they made this piece of gear. It is made for testing crystal diodes. A crystal diode is much older as the ” normal” diode. The first radios used a crystaldetector. A crystal that was clamped in some fixture and probed with a whisker. The 1n21 and family were used for (for those days, WW2) very high frequencys up to a few GHz. The crystal was packed in a ceramic housing together with a springlike needle and adjusted for a goed forward – backward ratio. The reverse resistance must be 5 to 10 times the forward resistance. A multimeter in the diode function will not detect a almost worn diode. So this tester was made for it. First position: set the needle full scale, second and third measure forward and backward resistance. Then the meteer must be set again and the next one is reverse leakage current. I still use it to test diodes for my HP and GR detectors and slotted line.