Well I said sandpaper, but it might been something like one of my polishing wheels, wouldn't leave a mark in silver that you could see but it will cut stainless.
I can accept any explanation for the rim actually because who really knows? Where it comes to down to
PMD is one side is plated, one side is peeled. That led me down the path that barring some magic at the mint I've never heard of (quite possible) that the rim is likely part of the same
PMD. If it had been struck like that, I'm pretty confident we'd see distress elsewhere, especially in the beads.
If anyone has any other ideas, by all means, lets hear them - even if you just want to chew at my speculation, it's all good.