As for the minor varieties, well, you can fill an album with what's out there.
That is a very true statement. The section I posted for Poland lists 15 known minor varieties. French's Encyclopedia has over 2 pages devoted each to Luxembourg and The Netherlands just showing the subtle differences in the background marks in the black shading of the flags, giving us Types 1 through 5 for each. (He references Clark Crane's definitive study.)
Unfortunately, as far as reverse printing, I still have not seen a good reference book with definitive pictures.