One of the most important aspects is whetherthe fretboard extension is glued ontothe top, or whether it is above the top.
I think that any with the fretboard on the top (as you would see on a flat top) then it is a more budget model - which are L xx, whereas ones with raised fretboards tend to be L-x (L-5, L-4 etc).
See:
http://www.guitarhq.com/gibson2.html