I've got two more possible candidates to nominate for your list.
What about Helmar backed S-74-1 white silks. You can find S-74 silks all over Ebay but, I've heard you can probably count the number of Helmar backed S-74-1s on the fingers of one hand. There are also Red Sun backed S-74-1 white silks which are probably less than 1% of the total number of white version silks out there but, there are likely hundreds, if not possibly a couple thousand, that are out there so they probably don't count.
My other candidate would be the 1921 Herpolsheimer's cards. They're basically the W575-1 blank-backed strip cards. Though I believe there are something like only 68 of these cards known to exist in total, they are limited to only one known example for each different player. Your call on that.
Bob
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