A. adonis is distinguished from other Acanthicus by having the white spots as a juvenile (easiest way to distinguish here). A. hystrix also has a more rounded snout and larger/more odontodes in this area. Also adonis are generall "less spiny" along the spine. A. hystrix also said to brown out a bit when they are older and large, whereas adonis stay more true black. When they are older is when people start confusing the two, but as Liz points out they do have different adult sizes.
A. adonis doesn't have an L number because it was already described by science, so didn't need to be catalogued.