They don't have one.
A Short history on L's numbers: First, L's numbers didn't exist until the 90's. Second: Not all plecos have L numbers. In theory once a pleco is identified that had an L number... it's suppose to 'Lose it'.
But many in the hobby refuse to give up the L number... so it sticks around.
Just like the Zebra pleco is well identified as Hypancistrus zebra but it's almost impossible for people not to kept calling it L46