My best guess is in the late 1990’s MAK Magic put out Mint Condition. This is an expanded shell coin that had the back of the coin on the inside. That allows you to show both sides of the shell when it’s empty.
I’ll admit that when it came out I thought it was a dumb idea. It was an improvement that didn’t really improve how a shell coin is used. For most close up magic, I still think it’s not necessary.
The place where this coin is great it for social media videos. Where is someone knows about a shell coin, it removes that when you show both sides of the coin. Social media is a great place for things that correct little details in magic tricks that are overkill for an in person performance.
The other thing is that I don’t think that Mint Condition was a huge seller, as a there’s not a lot of mentions of it when I do a Google Search and I can’t recall seeing one pop up on a used magic site. I guess them being uncommon is good for people who are using them!
-Louie