Where did "LL 928" and "LL 929" come from? Just finished putting Benny's Spaceship set together, and the Number printed on the side is "LL 929" which appears to be an increment from a much older set the Galaxy Explorer, which had "LL 928".

My question is whether there is any known source for this ID? Is it just something random that someone came up with or does it actually refer to something (real or made-up)?
928 was the original set number of the Galaxy Explorer - so the numbering of that one is following the LEGO tradition of using  the set number.
As Joubarc points out, at that time the sets were branded LegoLand, which is where the LL comes from (in recent years LEGO often uses the Designer's initials).

I'd say that as Benny is a rather well used Classic Spaceman and his Spaceship is clearly a throw back to that style, his ship is numbered in homage to the Galaxy Explorer.