I understood that. You ask when was the first time the European Commission has been called a "Guardian of the Treaties". I don't know the answer to that but I think that Schmitt's book-title may have played a role in it (esp. in its english translation), that is: A term used in constitutaionl law may have been borrowed to describe the E.C. Perhaps seen as a process of borrowing a term may provide further clues for your research.