People have been asking so I thought I'd post this.
Always excited to see something so rare and FF related become available. EBay has one of the FF Proofs listed. Item 381493091121
While the seller lists it as '1/1' etc, it isn't but rare nonetheless. What is it? It is a third of a 'missing color' or 'progressive proof' set. To date, I've seen maybe 3 of these exact FF proofs. 3 full FF proof sets actually. Not 1/1, but very rare.
Where did it come from? Probably either smuggled out of the factory by an employee or found in a nearby dumpster - discarded trash. Probably cut from a full sheet.
I believe it sold for around $300.