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?
I used to have one. It is more durable than a cardboard one and is more likely to be flat. Otherwise, just a strobe disc.

Too bad it is a 50hz disc. no good in the USA. You would have to be in Europe, or some country that uses a 50 hz A/C supply to actually work
Hi Elizabeth. It is reversible, with the "flip side" providing 60 hz! The 2nd photo in the ad shows the 60 hz side of this gizmo. Also it works with 33.3, 45, and 78 rpm records.
Dual included a aluminium strobe disc for years with their turntables that didn't have a built in strobe and had adjustable speed (1209,1218 ,etc.). Used one of these for decades until it finally became so scratched up that that I had to replace it. Bought the KAB strobe which is so much easier to use and more importantly to explain/interpret.Certain customers with OCD tendencies could get very concerned about the inherent inaccuracies in standard strobes -essentially printing error/centring issues.KAB is quick -works in any light and gives you the ability to check multiple speeds. Because the LED in this unit operates at 120hz from a battery - doesn't care about line frequency issues.
Great!! So . . . , after you learn that your TT speed is off, what do you do next, especially if it's belt driven??
Adjust it. I have several belt-drive TTs with speed adjustments.

Belt thickness. The speed relates to the middle of the belt. So speed off, get a thinner or thicker belt. Thicker would speed it up, thinner slow it down.
Do NOT forget: measure the speed with an LP playing!! the Lp playing on many belt driven TTs WILL slow it down a bit. So if you check it only with no Lp playing, it may run slow when it IS playing a LP.
This does not happen (usually) on direct drive TTs.