Courtesy of eBay, I acquired a pack of what seem to be called ELF cards. They have no indices (so are 1800s or earlier), and measure exactly 30mm by 45mm (so are probably not British). There were 53 cards in the deck, with an inexplicable duplication of the 9 of spades:
This is what I bid for, but the order actually came with two other components. One was a twentieth-century pack of miniature cards measuring 1.25 inches by 1.75 inches that I have no interest in, which is just as well because despite its being described as "complete" it was missing the 5 of spades, 3 of diamonds, queen of hearts and the 7 to jack of clubs.
The other component was a pair of playing cards (one, eerily, was yet another 9 of spades). I wouldn't have been interested except for what they had written on the back:
Oh wow! If I can find the other 50 cards, I am so holding them to that!