Where did "LL 928" and "LL 929" come from?
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.