Premise: "We're an American Band" (from the album of the same name) became Grand Funk Railroad's first #1 single on September 29, 1973, Mark Farner's 25th birthday. Written by Don Brewer and produced by Todd Rundgren, its huge chart success broadened Grand Funk's appeal. It was sung by Brewer rather than Farner, who usually took lead vocals.
Hypothesis: Mark's birthday was the same day their hit single was released.