I'm surprised no one has mentioned the D355 Niagara Baking back.
I owned the Otto Knabe, which I subsequently traded to Dan Gannt, who sold it in his divestiture over the past ±2 years. I don't think I've ever seen any other D355 example in the last fifteen years.
The E104-III Nadja's are certainly rare - but I'm not sure if they're that much rarer than E104-III blank backs. Which, if that's the case, I'm not sure becomes a valid comparison. (I've owned seven or eight E104-IIIs, and have had a fairly even mix between blank and Nadja back)
A bit like OP, I am the proud owner of a Pirate and Brown Lenox back - two cards that are unlikely to leave my collection anytime soon.
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