I looked at the Silver Slipper chips again and they still appear to be Silver Hotstamped. Also many of the other styles of roulettes from there are Silver Hotstamped. How about this theory - "Sliver" hotstamp = "Silver" Slipper ?? Someone could have odered the chips with "Silver" hotstamping to make the Slipper in the design truly a "Silver" Slipper. This would make sense to me - check it out.
Take care,
"Mr. Roulette"