What is an L-number?
Over the last few years, the hobby has seen an increase in the numbers of loricariid species offered for sale.
Many new ones have been discovered, and it is proving very time-consuming for the taxonomists to fully identify the fish and give them a scientific name.
To overcome this, a numbering system has been devised that gives these catfish an identity until they are described by science. As a result, you will see many of these beautiful catfish offered for sale with an L number, or an LDA number, with the L referring to the fact that the catfish belongs to the family Loricariidae.
From the Practical Fishkeeping website