The Imperial Library seems much more plausible on this point:
* In my first arm, a storm. - A Great Welkynd Stone
* My second, the rush of plagued rain. - A Great Sigil Stone
* The third, all the tinder of Anu. - The blood of an Aedra
* The fourth, the very eyes of Padhome. - The blood of a Daedric Prince
It's instructive to look at the
Commentaries along with the Mysterium: it is a mystery to me why Martin did not ask for the Commentaries (unless he already knew them?). The Commentaries mention two different "first keys" and no "second key", so it's possible that the second one is actually the second key.
Seek thy pocket now, and look! There is the first key, glinting with the light of a new dawn.
Feel uplifted in thine heart that you have this first [second?] key, for it shall strike high and low into the wormrot of false heavens.
This is the third key of Nu-mantia and the secret of how mortals become makers, and makers back to mortals.
Myth is nothing more than first wants. Uhokerrable truth. Ponder this while searching for the fourth key.... As God of the Mundus, alike shall be his progeny, split from their divine sparks. We are Eight time eight Exarchs. Let the home of Padomay see us as sole exit.
These are consistent with the interpretation given in the Imperial Library, and whatever interpretation you found on UESP makes no sense in relation.