A prototype chip made in several colors / inserts. Yes, as seen on the MGM Grand page in the ChipGuide.
Note the BALLY'S house mold -- Was there an ownership connection between the 2 properties at that time? (Not sure).
But the monorail connected them.

The Wizard of Oz design was not for a specific event, IMO. Instead, it was an extension of the interior design / theme in the early days of the property. See postcard of interior --

I have one of these, and there are a small number out there in collector's hands, but still a difficult chip to find.
Congratulations on adding it to your collection.