I built this a long time ago and have used it so much that I'd actually worn out the connectors by inserting thousands of LEDs into it over time. I originally built the unit to compare LEDs from different manufacturers for beam angle, output pattern and intensity. I was buying LEDs from various suppliers and using the device to choose the best batches for prop-making applications.
The unit has two Molex style connectors on the front to accept the leads of LEDs, and a two way switch to choose which will be powered from a PP3 9V battery and 270 ohm series resistor. There's also a 100K resistor across the two channels, so that if you put an LED in a single socket and turn that channel off it will glow dimly with just 50uA of current. In the case of gallium nitride LEDs this test will often show problems with the thin film light emitting surface. This could be due to a manufacturing defect or static damage due to improper shipping in ordinary polythene bags. With a damaged film the LED will not glow at low current, but may work again briefly if switched to the high current setting and back again.