Posted By:
MarkDitto on Adam's point.
I like to think that they survived far less than regularly printed cards and rarely available in the "market" so to speak; as many fell to the printer's eyes and were eventually disposed of or maybe past collector's dislike of the appearance of the card had them slowly "dissolve".
I have some scrap-like (badly miscut) E90-3's, a blankback proof-like T207 (lacking gloss on front), a really off-set T207 (blue print of NY logo is in the foreground), a badly off-register R300 Geo. Miller and a E92 Dockman/Menagerie wrongback...pretty unique, in my opinion.
The cards may not have a high value in the guides, but then again, the guides are usually off anyways.
Mark