Although I'd agree that it is unlikely that a printer would impose
such a label, they did exactly that with vertical title-labels. As
many of you will recall the function of these title-labels was to
enable a printer to identify stacks of folded, even sewn, sheets in a
warehouse (Brian J. McMullin lists many examples in his article in the
Harvard Library Bulletin: Title-Labels in British Books). To fulfil
this purpose the labels had to be easily read at a distance, and were
often printed in truncated form in a huge font.
It seems unlikely that a printer would do the same -- that is, impose
and print -- a single small label. Apart from anything else it would
have been seen as a waste of paper. Even in 1785 a blank page would
usually be used for something else; advertisements, a cancel, or the
title-page to another book. I think Edward is mistaken about a printer
having any great reluctance about cutting a leaf from a signature,
bound or otherwise. It was very common and does not seem to have
compromised the integrity of the signature concerned.
So it seems likely that, if the note refers to a small printed label
to be affixed to the (spine or cover), or a spare of the same, that
such labels would have been printed together and individual lables
clipped off and tipped into each book.
In my experience, with more recent books admittedly, such spare
binding title labels were always tipped in, not sewn; and so it is not
hard to imagine that, if the glue failed for any reason, the label
would simply fall out.
Patrick
David Klappholz's comment seems quite correct about the manner in which such
labels were supplied. In my experience, too, they tended to be cut sheets;
sometimes the same as the page size, sometimes smaller; sometimes sewn in,
sometimes tipped in.
Although I don't doubt it may have been done, I've never seen such a label
imposed into a signature (which would probably leave more clear evidence of
its removal). I would think most printers/binders would have taken a dim
view of supplying the label as a signature leaf; cutting such a leaf from a
bound signature could compromise the integrity of that signature.