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.
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.
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.