With the MVC architecture the underlying model was always the knowledge base which was a meta-model description of the specification and implementation languages.
The first metaobject protocol was in the Smalltalk object-oriented programming language developed at Xerox PARC.
The Art of the Metaobject Protocol (AMOP) is a 1991 book by Gregor Kiczales, Jim des Rivieres, and Daniel G. Bobrow on metaobject protocol.