Perhaps the earliest published appearance of "L7" as a synonym for "square" is in the second verse of Sam and the Shams' "Wolly Bully":

Hattie told Mattie
[???] take a chance;
Let's not be ell-seven,
Come on and learn to dance;
Wolly Bully, Wolly Bully

Also, not too many years ago, I think there was an American rock band ~ maybe an all-girl band, if I'm not mistaken ~ called "L7."