imposing with an imposition question

[The Prints & The Paper <theprintsandthepaper@xxxxxxxxxxx>]

Imposing on your goodwill! A fellow bookseller has come up with the
following question, which has been batted around but not definitively
answered. I thought perhaps someone here could help out! Thanks, Lee

The question:

I'm looking at a 1st edition of Master Humphreys Clock by Charles Dickens
and I'm wondering about how to describe it. Page size is 9 3/4" by 6 1/2"
which one would expect would make it either an 8vo or 4to - well I would
expect that anyway. But when I look at how it is signed I find that each
gather has just 6 leaves. I had a quick look at Philip Gaskell's
'Introduction to Bibliography' but couldn't see anything in the imposition
diagrams that seemed to match this and I'm unsure how to describe it. I
actually need to reread Gaskell and I'd recommend it to anyone interested in
the production of books, particularly those produced before the
mechanisation of much of that process - who knew paper could be quite so
interesting.

It just occured to me that this format could be achieved by folding a sheet
into thirds (2 folds) and then folding the sheet in half. So like an octavo
the sheet is folded 3 times but you end up with 6 leaves instead of 8 but I
still have no idea how to describe this.

                                    
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