The clay chips were fitted with metal rims because the clay edges couldn't stand up at all to the abuse of slot machine hoppers. Tokens need to be very consistent to work in coin mechs. I suspect the chips were milled down due to some maximum diameter of the $5 coin acceptor mechanisms. Milling them so the outside diameter was the same as the original chip would have taken away a lot of the "chip-ness" that they were obviously looking for. Coin Mechs of the day had rockers, magnets, etc. to accept or reject tokens based on diameter, thickness, weight, magnetic content, etc. If you made your $1 tokens from different materials from the casino next door, your slots could reject the foreign tokens even if they were the same size and thickness as yours. You could make a $5 token the same size as a $1 token but different enough that a mech designed for one wouldn't take the other. One advantage to having different sizes for different denoms is the ability to easily see which tokens are in a rack. This also gives the player a sense of greater value, same as with regular coins. $5 tokens the same size as chips have the advantage of fitting 100 to a standard chip rack. I could go on, but I won't!