On the IPDB, there is a photo of the castle insert area of the playfield (shown below). The castles are surrounded by a blue border on the MM prototypes, while the production versions have white borders. Other than that, I don't know of any other differences between prototype and production playfields.
There were some other non-playfield related prototype/production differences discussed a while ago (maybe 12 years ago) on the RGP forums, like how the prototype MM's didn't have the plastic triangle shaped guides on the floor of the ramp entrances, and about a possible wider set of dragon wings.
I don't know how many prototypes were made.MM prototype.jpg