L stands for Loricariidae, the family of Suckermouth Catfish in which plecos are taxonomically classified by scientist. Soulfish has it right, so many new types of Loricarridae are being found faster than they can be studied and classified by scientific genus species names.
Also, many plecos are given absurd common names that are used by laypeople in the aquarium trade. Different species are given the same common name because only a truely discriminating eye can tell them apart. That's why you'll find, for example, Gold Nugget Plecos carry several different L-numbers. Genus species is the most accurate classification, L-numbers the second, common names the last.
~~Colesea