In Riemannian geometry, the "lowering indices" operator is denoted by $\flat:TM \to T^*M$ and the "raising indices" operator by $\sharp:T^*M \to TM$. These isomorphisms are sometimes referred to as musical isomorphisms, as stated on Wikipedia and in several other sources. Surely, the motivation for such terminology is clear. I would nevertheless like to know who decided to adpot these (rather amusing) notations, so here is a question:
What was the first paper / textbook that made use of the notations $\flat$ and $\sharp$?
and a possible follow-up question:
If such notations were not adopted widely after the first appearance, who popularized them?