Fair questions...there are a few id keys to diff between L418 and L191....
1. 191 are also known to be dull-eyed, so the first thing I look for in a L418 are bright orange eyes...all things being equal, this is the most obvious difference.
2. of course knowing the river source or shipment origin helps, not fool proof but knowing this fish came in from Peru settles it.
3. L418 appears from the top view, vis a vis a L191, to be more slender and streamlined, the L191 is more tadpole or ruby shaped.
4. Fork versus paddle tail, you are spot on, I am not sure if this is age related as this is the first time I have a 4"TL juvenile. However when I first bought this piece, it has a big chunk of caudal fin beaten off, it has grown back most of it hence the "fork" which actualy is the hole.
Anyways knowing the above all id keys (plus the price! cost me double the price of a 191) settle the id for me.