From what I've gathered, Mythos is the uncensored version, for institutional use, and then Fable is the censored version for general public, that won't talk about biology, encryption or anything remotely interesting

The first number is which generation of their LLMs it belongs to.

Fable is the first model in the 5th generation.

The second number is an incremental release, not a generational leap forward.

It seems it is just like macOS releases, they have a number and they give the numbers arbitrary names to refer to them?