I believe this is from the 1968 San Antonio HemisFair.
Clear plastic. Painted wording. A business sized card can be inserted on the underside to provide a background for the wording. I have had this for quite a while and was not sure exactly what it was. Finally decided it was to be used as a coaster as the round area is just about right for a 12 oz. bottle or perhaps a shell glass. The two horizontal lines within the circle are raised ridges which would aid in preventing the coaster from stckig to a wet bottle.