I think eyeonwall is on the right track, but I don't understand this part of his response:
"so the bottom half of the die pair git inked."
So, replacing that I would insert: When the previous envelope blank didn't get fed properly, the ink on the die was deposited on the platen. Then the OP's blank was fed and an offset image was printed on the underside of the blank (ink coming from the platen). The other side of the OP's envelope may have a colored impression or it may be an albino, depending on the scenario.
While these are not rare, getting one with just the right amount of ink (i.e., visually pleasing) gives you a range of valuations. The inked impression will attenuate after the OP's envelope was printed, getting fainter with each subsequent envelope coming through the press.