Some chips have a metal weight in the center of the chip. Sometimes it shows through the inlay as in the scans you show.

I don't think that's the case in the Riviera 50c as it's not centered and too small.

Also, a cancel would have to break the surface of the chip and that's not the case in the Riviera. The round white area I mentioned on one side of the chip was obviously made when the inlay was produced. Likewise the printed casino name.