I would think it is a 10 frame deep box with a 3/4" divider board in the center, which splits the deep box into 2 - 4 frame sections side by side, giving a double 4 frame nuc box.
I have also, as you said, some 4 frame nuc boxes that do not have the bottom board built in. I can stack them up like regular hives but they get a bit wobbley from being so narrow. I also have some 10 frame queen excluders that I've split in half, each half will cover a 4 frame nuc box. So I can stack my nuc boxes to make a tall skinny 8 frame hive, and have excluder to use if I want.
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