Yes I've already consulted that (no offense) for my library at least 10 times.

What's not clear to me is how they choose the version bit hex characters so that they align with the base58 characters, because as you know, 58 is not divisible by 16 so there will be some spillover into the 5th Base58 extended key byte.

Another trapdoor is whether stuff is encoded into base58 starting at the beginning of the string or the end of the string.