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JimB"However, how many uncut sheets are out there?"
To my knowledge there are two uncut sheet complete sets that are actually cut into strips. Mastro auctioned them both last year and they both came from the same source outside of the hobby. There is also a partial sheet that Goodwin has uctioned twice in the last couple of years. In addition, there is one complete sheet of the entire set which is uncut and is apparently in mint or close to mint condition. It is in an advanced collection and though I have tried a couple of times, I don't think it is moving any time soon.
"I've seen two for auction within the past year and there are undoubtedly more which have been cut over the years. I'm quite leery of these blank-back "proofs" which show up on ebay and am equally as hesitant at the more frequent high-grade examples which have surfaced. "
All of the blank back examples that have been on ebay in the past year came from the lot of 19 that Mastro sold last year. The buyer has simply broken up the lot and resold them. I do not know of any other authentic E93 blank backs. These all came from the source of the uncut sheets and apparently were from a third sheet that at some point in the past 100 years was cut up.
I do not know what frequently surfaced high-grade examples you are referring to. There are barely enough high-grade examples for two complete sets at ex/mt or higher. The second best set may not even have all cards at ex/mt or better. Beyond those two sets there are not many more out there. These high-grade cards have not come from these recent uncut sheets since all of the sheets were blank backed.
JimB