This is a patent HMV device to check that the turntable is revolving at 78 rpm. The unit it placed over the turntable spindle and when turning at the right speed, a small gold disc on the top sits level with the top of the unit. The gold disc rises up if the speed is too great and sinks below the surface if the turntable is spinning too slowly. A more modern method to check turntable speed is to use a stroboscopic disc, but this relies on viewing the disc in house lighting flashing a 50 cycles per second. I am not sure that modern lighting with LED bulbs would give the same effect?
This tester is in mint condition and is complete with its box and instructions. £75