Origin of the slang "L7"

According to a Reddit post

A square.. hence shape of L7 {}

the origin is that the two adjacent characters L7 looks kind of like a square.

It doesn't look very square when the riser of 7 is on an angle (as in most modern computer fonts), but if you write it vertically it's pretty close.

I found a number of references with definitions (Urban Dictionary, Dictionary of Slang) but they didn't offer origins.

Its origin appears to be from teen Black AmE, from the ‘50s. Is it used both as a noun and also as an adjective:

L-7 (noun) also l-seven

[the L and the 7 when put together form a SE square thus a pun on square n. (3b); the word can be accompanied by using thumb and forefinger extended at right angles, forming an L and a 7, and when the two are combined they form a square]

(US black/teen) a conventional, tedious person, unsympathetic to teen interests.

• 1956 Artie Shaw in West Coast Jazz 79: Hollywood’s latest lingo for a square: for an L and a 7 with your fingers and that’s what you get [W&F].

L-7 (adjective) also l-seven

(US black/teen) unfashionable, unsophisticated.

• 1972 D. Claerbaut Black Jargon in White America 71: L7 adj., completely out of style; not like the group.

(Green's Dictionary of Slang)