You are correct.
In 1973/74, Manchester United finished 21st of 22 in the old First Division and were relegated. Nevertheless, they topped the attendance charts that season with an average of 42,712.
In 1974/75, they won the Second Division championship. They were the best-supported side in the Football League with an average attendance of 48,387. First Division runners-up Liverpool were ranked with an average of 45,966.
In 1975/76, United finished third in the First Division and were comfortably the best-supported side in the Football League. They averaged 54,750, while champions Liverpool were ranked second with an average of 41,623.