the L-numbers are a system from the german magazine DATZ who tried to put a bit of order in the explosion of "L-Welse" (Loricaridae catfishes) in 1989.
there was so many L fishes coming on the fishhobbyist market that scientists couldn't publish every week an official describing article. So Rainer Stawikowski (editor in chief of DATZ) put in place the "L-Numbers" system, considering in placing a L-number (beginning with L001 -> later described as Pteryglopichthys joselimaianus) on every new article he published in his magazine, one after the other, to recognize them...
in 1992, Das Aquarium copycated the system to extend it on other generas, with the LDA system...
Talking about the L27's (a-b-c), it's all because this fish was "published" in one article, but with 3 origins mentionned... Panaque cf nigrolineatus "Tocantins", "Xingu" and "Tapajos"...
From this time, Panaque "L27" has been described :
L27 a is Panaque cf nigrolineatus
L27 b is Panaque armbrusteri
L27 c is Panaque cf armubrusteri "XINGU" (aka LDA 63)
a bit difficult to follow, as everything when scientists (they are only humans) want to be the last to be "in the tablets" and figured to have "THE" truth of classification
