I heard that it was Billy Grammer who thought of and did the tuned note guitar lick in "Detroit City" by Bobby Bare. I don't remember any links to prove this.
Based on the post about Charlie Mckoy saying that he worked with Jerry Reed on the tuning peg note, I'd say that Jerry Reed and Charlie Mckoy produced that note and probably the tuned up note in the middle of the great song by Bobby Bare Instead of Billy Grammer.