BC Wills and Co. internally called the Large Greek Key mold the "Super Gem".

Also, as Roland points out, there is a listing of a yellow, 50 cent chip, for the Horseshoe in Reno, in a Large Key mold and that the gold hot-stamp is of a horseshoe.

I would also assume this to be a final version of the chip you are showing.

An interesting find to be sure!

Jim