More from inherited 'stuff' that I don't know what it is.
This is made from thin, stamped, metal. It is 6 1/2" diameter with a 1/2" arbor.
Wattizzit, y'all?
I'm pretty sure it's the bottom separator disk from a spindle of real old blank CDs...back in the day when they were made of wood, and computers were coal-powered.