No one could fault you if you don't remember the Surface Music Kit. Announced alongside the ARM-based Surface 2, it was a Touch Cover that was designed for remixing music, and it never made it to market. It was part of a broader initiative to produce different styles of swappable keyboards that serve different purposes, called "blades".
But while the Music Kit never saw store shelves, some were handed out as part of contests, or for various other reasons. Whatever those reasons may be, they do exist, and you can now purchase one on eBay, as a Reddit user has spotted.
Indeed, for Surface fans out there, this is a part of Microsoft history, and even if you're not going to use it, it's certainly a collectible. If you are going to use it, it works with the Surface 2, Pro, and Pro 2, assuming that it's even still supported.
The price seems to be going up though. When Windows Central spotted the post on Reddit, it was $11.99, and it was $12.99 when we bought one. Now, it's up to $13.99, which is still a really solid price. It's unknown how many of these are in stock, but there seems to be a bunch.
And yes, we totally bought one, and will be unboxing it as soon as it arrives.