Greg S. answered 09/14/19
Music Lessons: Theory, Songwriting, Performance | Online or In-Person
Indeed, a letter by itself means play the root position. And I don’t know for sure how the current method got started. But I suspect it grew from the lead sheets of the big bands guys. And I think it was in the 50s and 60s, with Nelson Riddle and Burt Bacharach (among many arrangers), that the symbols became standardized.
It you find a better story, please let me know.