The "L" numbers were created because there were a lot of fish coming out of the Sth American jungles that did not even have scientific names, and we all know what a mess Common names are for fish.
So they were given "L" numbers so at least there was some consistent reference for them. Many now do have a real scientific name, but the L numbers system is still a pretty convenient way to reference them.
Here is a list of the L numbers.
http://www.planetcatfish.com/catelog/numbers.php?mode=l&thumbs=16
Ian