All Mounting-Plates are designed to receive the Upper or Lower Bases. They come in three sizes (small = blue, medium = green, large = orange). Three views of Medium Mounting-Plates are shown above. The surface opposite the snap has undercuts built in around the inside edge of the depression to insure that it stays attached to the plaster when mounted on a fixed articulator.
Above pictures show a Medium Arch Mounting-Plate used on an upper and lower Medium Quadant RTP_Base. Since both models utilize a Mounting-Plate, it is very easy to unsnap either model and return it to its locked position.