a few thoughts
1stly they are not proofs, they were obviously issued and circulated and found in old time collections. While not know for sure, the B & W ones were probably issued by some other party than C.A. Briggs and purchased from the same lithographer, they were just on a budget and went B & W and no printed back, just a hunch of mine.
The pictures are identical lithographs of the full color E97's. The cards are on much thinner stock.
They are not rare, perhaps scarce is appropriate. They used to command 4 figures for even g/vg examples but can now be found in the mid 3 figure range for that grade or nicer(however g/vg is about the best you will find). We have auctioned a couple in the last year.
Scott
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