The black markings remind me of the Mirage 5000 NCV chips -- "Series 18" #CG-321608 (see pic below, from MoGH/ChipGuide):

This Mirage chip is part of a 5-piece set that's recently come out in quantity. Used in tournaments as described. My speculation is that some of the unmarked 5000 NCV's were "removed" from earlier tournaments, so the remaining ones were marked with black permanent marker (as noted in the ChipGuide listing) so the unmarked ones couldn't be rung into later tournaments. You can barely see the 4 marks on edge, but it's there, including in the one I recently bought.

So, it could have been marked for a home game, or perhaps used for a Crystal Bay tournament or other purpose? (Did they even have tournaments?) Hard to say.