Oh, but it is ZK furniture in every sense!

Then almost everything is "zettelkasten" in that sense. I go by the narrow definition here, because we're talking about zettelkasten, the practice, not zettelkasten - the word. But if we want to go by the word, then I guess we can stop talking about Luhmann, and start worshipping index cards. We can call ourselves the "zeeks", and walk around in long robes. Nah... I jest. ;)

but I'm sure there was a subset of customers, particularly those in academia,

Academia gets into everything, so I'm sure there were some many who adopted it in their own usage - likely influenced by how they were being used in the "real world", as it were.

this would likely have accommodated 3 x 5" index cards

Who knows? I think that's really small and makes the desk really low to the floor. But you would have a better idea than I; more than likely you have looked into this more closely than I.

But hey, here's a beautiful restoration someone did of a Moore’s Cabaret Desk 1878; https://www.naples-furniture-restoration.com/images/p2157468-jpg.jpg

few seem as interested in the historical aspects

Yeah, I really like to read about the historical aspects because they can aid informing us about the culture of these practices as they began to use it for themselves. I'd be interested to hear more about Konrad Gessner - I'm probably going to spend a little time with Google and ChatGPT on this.