As juveniles I think it it quite hard to differentiate between the two but better pics are needed, focussing on the head.
As for L600 as an L number, it wasn't issued by DATZ, I think it was coined by exporters. But I think that it is better just for ease of use for it to remain used as an acceptable name for Pseudacanthicus leopardus. As with other L numbers, it takes on a life of its own within the hobby.