MDL was a dialect of lisp invented by/in part/under Sussman, the originator of Scheme and SICP; what you're calling an obscure dialect was was part of the continuum of a research trajectory, one of a number of experimental languages designed to test out ideas. Sussman got his PhD in 1973 so we're talking about his later work as a student/early work as a postdoc/assistant professor, and Abelson was in the same timeframe, and Guy Steele a half decade junior, and many others in the lab whose names you would also recognize.