In fact M-exp where intended for Lisp code before S-exp (which were originally meant to represent data only). IIRC, the story is data S-exp were used for code t

In fact M-exp where intended for Lisp code before S-exp (which were originally meant to represent data only). IIRC, the story is data S-exp were used for code temporarily because it was easier this way, and then they find it usable enough.