This observation has been on-going since the 1950's. In particular, when it was first suggested by Baench. Its long since been discounted by science. With the exception of Pygopristis denticulata which has a bilobed anal fin, the shape is an individualistic trait. What that means is, S. rhombeus 1) like humans that have individual traits (long noses vs short noses, color shades, etc.,), so it is with the anal fin. 2) Add to this the fact they are fin biters and the anal fin being one of the areas frequently bitten, the regrowth will vary in size by the number of bites the fish receives. In other words, it doesn't always grow back the way it originally looked.
I still find remarks about thickness of first ray, length of anal fin, etc., being discussed by web sites and forums being an indicator for sex determination (sexual dimorphism). Unfortunately, it's an observation that is from the uninformed who think they've "discovered" something.
Thanks for asking.
