I found this random pastebin with apparently all of the unique NPC weapon names: https://pastebin.com/SwzeURQJ
As mentioned before, WEAPON_REVOLVER_CATTLEMAN_LENNY will show up as valid in the trainer menu but does nothing. I also tried WEAPON_REVOLVER_DOUBLEACTION_JAVIER, which did not show up in the menu. I'm guessing none of the unique NPC models can actually be used by the player, with the obvious exception of WEAPON_REVOLVER_CATTLEMAN_JOHN, but there they are.