Thorens Turntable Speed Indicator Accessory

Perusing ebay I noticed this unusual accessory made by Thorens. I like the concept that this speed indicator, made from aluminum, is built to last unlike the cardboard indicators I’ve previously used and possibly it is more precise.

Just one of those curiosity things for me especially as Internet searches resulted in a complete absence of information on this particular device, surprising as Thorens products are so popular. Anybody seen one before? Any guess when these were available, 1950's, early 1960s?