Signature Key for a REPO (Linux MX)

Sillily (LOL) I tried a Dist Upgrade  (Linux MX dist upgrade from Wildflower to Libretto)  without following exactly the procedure in the MX 23 Forum
here :
https://mxlinux.org/wiki/system/upgrading-from-mx-21-to-mx-23-without-reinstalling/
I resume the steps below
Upgrade Steps:
1)    Upgrade your system to the latest packages with:
sudo apt update; sudo apt full-upgrade
2)    Install mx23-archive-keyring:
sudo apt install mx23-archive-keyring
3)    Change sources to point to bookworm release (search all .list files and replace “bullseye” with “bookworm”):
find /etc/apt -name "*.list" -exec sudo sed -i 's/bullseye/bookworm/g' {} +
4)    Add non-free-firmware to debian sources (or by hand: edit the debian.list and add “non-free-firmware” to each deb line):
grep -qF 'non-free-firmware' /etc/apt/sources.list.d/debian.list || sudo sed -i 's/non-free/non-free non-free-firmware/g' /etc/apt/sources.list.d/debian.list
5)    Upgrade packages (make sure nothing important is removed at this step: examine the prompt before you press “y”):
sudo apt update; sudo apt full-upgrade
Now, acting sillily basing on what I recalled, I performed all stages FORGETTING 2)  Install mx23-archive-keyring:
The upgrade resulted in PARTIAL success (and failure overall). I dunno exactly what was upgraded and what not.
I then, late alas, to do step 2) afterwards, but then the package  mx23-archive-keyring  was no longer available.
Maybe it belonged to the ORIGINAL MX21 Repo, that had been overwritten by hand.
Am I right ?
Is it still a way (I tried using WGET, failing, as unable to locate the correct package name, which apparently is neither mx23-archive-keyring nor mx23-archive-key or suilar, and moreover the URL could be wrong too or both), but well, as I was saying : Is it still a way to install a single package from a given repo (the original WILDFLOWER 21 Repo) in the new system, and then repeat the upgrade procedure ?
Some core package, resulting NOT SIGNED, refuse to be upgraded, so such keys are necessary.
I hope I was able to explain sufficiently sth I don't understand myself.
Tnx for any advice for this mess (the system runs, and reports itself as the OLD one, even if it upgraded many hundreds of packages)
--
1) Resistere, resistere, resistere.
2) Se tutti pagano le tasse, le tasse le pagano tutti
MarioCPPP