[Malte Müller]

Could be! Plus, the stereo [Pet Sounds} is “artificial” (meaning post-converted).

Actually, it’s true stereo, it’s just that it had never been mixed to stereo until the 1990s.

That was my impression as well. Same with some for example older Beatles albums that also exist in two versions. To my knowldege the reason was primarily that they were already recorded with for steroe but since in the arly to mid 60s most people did not yet have stereo device they were (also still) released in mono.

In iTunes, if you add cover art to an album, it’s inserted into the files (on Windows at least). I didn’t know that it works differently on a Mac.

Yes, I re-tested it to be sure I tell it correctly. The current version does do it as expected. I could directly see on the Finder (the folder view) directly how the cover is added as the icon changes visibly then. But it was not always the case with older files I am sure. Might be because I naturally have files from various sources over the last 20+ years that might not be done correctly always… Maybe just like the download service have some weird tagging at times I almost always change… As usual, if you don’t do everything yourself… 😉

I only add new music to the backup, I don’t add all the small changes I do to the existing albums. I know, I should. But it’s too much work. I have too much music.

I don’t do this manually. I use macOS built in backup tool called Time Machine that does incremental backups, e.g. it looks for file changes and only backups those. Since that can only backup to one target, I also have another backup tool for manual backups which does the same.
I don’t know if or if not why Windows does not have such a tool. But there are solutions to not do such things manually for ages.