Although we never had a Claude Opus 5.1. Fable went 5 → 5.1 and Opus went 5 → 5.5.

I assume Fable 5.1 is the first re-tuned (is there a better term for this?) version of Fable 5 and Opus 5.5 is the first re-tuned version of Opus 5, and maybe the Opus one just came out better for some reason?

Opus 5.5 is probably a distilled version of a 6 model.