MTC21 was never intended to be a general DCC decoder plug format.
The letters of MTC = Marklin Trix Connector. A proprietary connector that supported DCC on a specific locomotives being made between these two companies.
The problem is the socket grew beyond what it original intent was and got adopted by other locomotive manufacture before any real standards could be defined for it. Hence the multiple versions.
Many thanks to the guys over at Streamline Backshop for the information