In the ‘olden days’ it was really cumbersome to copyright a musical work. Regardless of whether it was a full scale symphonic work or a simple three chord song, in order to copyright it, it had to be notated in standard musical notation. Only then could it be submitted to the Library of Congress for copyright approval. Once approved, it would receive the familiar ® or © (they're interchangeable) symbol used to declare the copyright status of the piece.
When I graduated high school, I earned a living by transcribing audio recordings (usually cassette or reel-to-reel) by composers, songwriters, arrangers, performers and even jingle writers. It was very exciting to experience these early versions of pieces - some of which eventually became well known. And it was great ear training practice.
Then in 1976 it all changed. Congress passed the Copyright Act of 1976. This new law (Public Law number 94-553) greatly reduced the hassle of copyrighting music. All you now have to do is submit a recording of the piece in almost any and all formats. That law also created a new symbol to designate protection, ‘P’ in a circle ℗. The ‘P’ stands for ‘phonogram’ which is another word for the phrase ‘sound recording.’ You probably never noticed but check out the symbol on a CD that was manufactured in the last few decades. It’s sure to be a ℗ not a © .