L7 was at least American slang for square or straight edge. Jessica Willamy, then an enthusiastic member of the counterculture movement in the Haight district of San Francisco, thought Ben Jackson "L7-square" upon meeting him in 1967 because of his hair and clothing. (PROSE: Wonderland)
Behind the scenes[]
The term derives from the fact that if you put an L and a 7 together, they form a square.