77 Jax proof overprint.jpg
The "Rarest Reggie" card, the 1977 Orioles proof card, is going to be a featured lot in the April 8 Collect.com Auction (bidding begins March 22).
Actually, the entire 66-card proof sheet, which includes, if memory serves, another five-six unissued proofs including Rollie Fingers, will be offered.
There are three or so singles of the Jackson known, but as far as I know, this is the only uncut sheet. At least it's the only one that's been publically offered since 1991.