L7 (meaning boring, dull) could come from one of two sources,

1. In the (say) 1950s folks would make hand-symbols in the air. (There’s a retro example of this in the movie Pulp Fiction for example.) If you do an L and a 7 it’s a "square" symbol. Could be L7 came from those "50s style" hand symbols.

2. On a (I suppose) computer, or just in paper typesetting, "L7" looks like a square. Could be L7 came from typesetting / letter shapes.

Regarding 2, one would think it would be more like only in the computer/SMS era (perhaps from, say, 1990 onwards at the earliest) – however it may have been based on plain writing or typesetting. Regarding 1, I really don’t know when "hand symbol squares and similar" was first a thing ("the 50s" was a wild guess; could be much earlier or later).

Regarding L7, the earliest reference I know of is

1964

in the song Wooly Bully.

Does anyone have any firm actual references on whether "1" or "2" is the origin?

or indeed an earlier reference?

Please note that there are any number of online mentions of BOTH explanations. For example on the "wikipedia" page, some person has typed that L7 is a hand shape. This is completely unsourced and, just someone typing. (Actually FWIW I’ll remove it from wikipedia just now, since it’s totally unsourced.)

You can find any number of mentions on the net of folks who think one or the other.

Does anyone have any sourced firm early explanations or references; or perhaps an earlier usage than 1964?

Note that for example Green’s dictionary says it is the (written, typed) letters L7 (but indeed they mention it can be "accompanied by" the hand shape). So unfortunately, absolutely no information there about origins or dates, only a description.