Hi Zayne,
as skippy_49nz says "the L number is assigned to a new species until it is described."
As for the more complicated explanation. I'll try to keep it simple and hopefully it will make some sense.
Basically in the early days (since 1988) there were lots of new types of plecos coming out of South America into peoples aquariums with no scientific names (they were new to science and the scientists hadn't had a chance to give them a scientific name). It was hard for fish keepers to refer to any one of these new plecos as they didn't have a scientific name or in many cases a common name, so a German magazine decided to call the first new pleco L001 (short for new Loricariidae (the name for plecos) number 1, or effectively new pleco number 001), the second one was called L002, and so on. There are now several hundred L-numbers. A few of these plecos now have a scientific name and many a common name or common names. For example L001 is what we now call the gold spot pleco, or to give it its scientific name, Pterygoplichthys joselimaianus.
To confuse things a little more :-? another German magazine decided to do the same thing and started the LDA-numbers. Luckily there aren’t as many LDA-numbers but some plecos do have an L-number and an LDA-number.
If this makes sense so far great :-? but what is also confusing is why do some plecos have a few L-numbers? This is because some types of plecos are found in more than one river system/river/creek, etc. and there might be slightly different looking forms of the same pleco in each different river or even in the same river. Each of these forms may have been given a different L-number (just to be safe) even though they are the same species.
So the clown Pleco, or Panaque maccus to give it its scientific name is also known as L104, L162, and LDA22. There are several webistes where you can look up the L-numbers. Planet catfish is excellent.