The two "L" numbers are the same species, Pterygoplichthys joselimaianus, according to all sources I have accessed. Here is the entry from Ferraris, "Checklist of catfishes, recent and fossil (Osteichthyes: Siluriformes), and catalogue of siluriform primary types," Zootaxa 1418 (2007).
Pterygoplichthys joselimaianus (Weber, 1991) Glyptoperichthys joselimaianus Weber, 1991: 640, illustrated in Weber (1992: 20, pl. 15b). Type locality: Rio Ara-guaya, affl. du Tocantins, système de l’Amazone, Aruanã, Goiàs, Brésil. Holotype: MZUSP 4873. Distribution: Tocantins River basin, Brazil (Weber, 2003).
The Ferraris checklist is available online for free:
Weber's description is here: Weber, C. 1991 (Sept.) "Nouveaux taxa dans Pterygoplichthys sensu lato (Pisces, Siluriformes, Loricariidae)." Revue Suisse de Zoologie v. 98 (no. 3): 637-643. [English summary.]
Weber described this species in the genus Glyptoperichthys as Glyptoperichthys joselimaianus, and it was moveed into the current genus by Armbruster & Page (2006) in this paper:
Armbruster, J. W. and L. M. Page 2006 (27 Dec.) Redescription of Pterygoplichthys punctatus and description of a new species of Pterygoplichthys (Siluriformes: Loricariidae). Neotropical Ichthyology v. 4 (no. 4): 401-409.
which is available online here:
I have not accessed Weber's paper, and the two "L" numbers might be mentioned therein. I would suspect that it was simply a matter of two different specimens collected by different collectors being assigned an "L" number. I am sure this must often occur. When the fish is actually analysed and described (and named) as a distinct species, this would come to light.
In the case of all "L" loricariids, and also the "C" and "CW" Corydoras fish, they are "discovered" and assumed to possibly be a new species and thus assigned the number for temporary identification. Only the formal description can determine if the fish is or is not a new distinct species. Geographic differences in thee external pattern of any species may suggest it could be a distinct species, but it may or may not turn out that it is.