Stanley Casinos is an outfit in England. Chipco did make a chip for them, so your assumption about overstamping looks correct.
No info on the Caesars chip. Best guess is that it's a sample/prototype roulette (?) chip for Caesars that wasn't made. Just a guess, but it might have been made about the same time that all the Caesars-AC chipcos were made, and Chipco was attempting to get some LV business from Caesars.