I post these too much...
But here are my top 3 unique "mystery" cards
Mystery Ruth Premium (either a prototype R309-2, or a R340?)
Unique, extra wide green border and green name plaque, better photo quality than the R309-2, different style easel. Some have said it might be a R340 Sport Kings Premium (unissued as planned, see wrapper scan)
ca.1941 D-unc. Kroger Bread
unique, never seen another...yet
postwar unique Aaron
discovered on eBay 2 years ago, unique, never seen another...yet
(different style than the more common HR Derby cards, players name moved from bottom to top and beer logo placed at bottom)