Sardinia Mix Of Forgeries And Legit Stamps

I picked these up recently, I got intrigued after finding perf12's breakdown on the forgeries. I plan on going through them in the next week or so and posting my determination on which ones check out and which ones are forgeries. One stamp though I can't find anything on so I wonder if it isn't some sort of essay. As seen in the photos below it's quite reflective with a bluish silver tone. The original hinger has it labeled proof? But it doesn't seem to match any color variations I've found. I'm assuming a forgery but would love your guys opinion.

This resource is very helpful if you have not already seen it:

http://stampforgeries.com/forged-st...econd-issue/

Yes that's the series I am referring to. The whole series of the different issues of sardinia done by perf12 is amazing