• DÉFETn. m.

XIIIe siècle, defect, « manque, défaut » ; XVIIIe siècle, au sens actuel. Emprunté du latin defectus, « disparition, défaillance, manque, défaut ». TYPOGR. Feuille ou fraction de feuille d'un ouvrage imprimé, qui reste en excédent après l'assemblage. On conserve les défets pour remplacer éventuellement les feuilles endommagées ou égarées d'un ouvrage
[CNRTL]

Défet : en conservation-préventive, feuillet ou élément quelconque dérelié du volume qui sera conservé sans être remonté dans le volume.

Here -
https://publications.gc.ca/collections/collection_2014/tpsgc... https://vitrinelinguistique.oqlf.gouv.qc.ca/fiche-gdt/fiche/...
https://www.btb.termiumplus.gc.ca/tpv2alpha/alpha-fra.html?l...
we find 'oversheet' for défet. It might do at a pinch but I remain to be convinced.

Getting there …
• défet [oversheet]
Feuille superflue et rejetée par l'imprimeur pour sa mauvaise qualité d'impression. Il peut s’agir également de feuilles ou cahiers dépareillés restants après brochage des ouvrages et avec lesquels on ne peut former des exemplaires complets.
https://www.lectura.plus/3143-glossaire-de-la-bibliophilie-c...

Also here: https://books.google.fr/books?id=nNePEAAAQBAJ&pg=PA330&lpg=P...


Allowance, overcount, oversheets, plus sheets - Additional paper required to compensate for sheets inevitably spoiled during makeready, start-up and production (waste)
https://www.koenig-bauer.com/fileadmin/user_upload/KBA_Repor...

Ichthyologia Ohiensis. B.M. Lexington, Ky. Separately published from oversheets, from same type, as the papers appearing in the Western Review and Miscellaneous Magazine, 1819-20 [ … ] American Manual of the grape vines. B.M. 12°. Philadelphia. [Reprint of the article on Vitis in Medical Flora Vol. 2, pp. 121-180, 1830. Made up of oversheets with eight drawings.]
https://www.euppublishing.com/doi/pdf/10.3366/jsbnh.1943.1.1...

Oversheets — Signatures or sheets left over after complete copies are gathered and bound.
https://www.survivorlibrary.com/library/the_printers_diction...
[The Printer's Dictionary of Technical Terms, 1912]

I'm convinced. Chapeau to those Canadian lexicographers.


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Note added at 16 hrs (2024-04-17 09:40:15 GMT)
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Theoretically a défet is, as I understand it, a sheet left over after a run of books has been printed. It might be an extra sheet programmed in the print to allow for replacement should any damage occur to a book during binding, or maybe its a sheet from a book too badly damaged to be sold. Also, these days, lots of old books with attractive illustrations are taken apart and the illustrations sold as framed prints, etc. while the rest is pulped.

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Note added at 2 days 7 hrs (2024-04-19 00:26:32 GMT)
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An interesting dilemma for the translator! The curator may have made a mistake in using a word that will undoubtedly be understood only by experts in the field of printing and bookbinding. An equivalent word exists, but should it be used, leaving the English-speaking visitor equally as perplexed as the French? Even if défet appears in my Lexis and Larousse dictionaries (but, strangely, not in Alain Rey's historical dictionary), I doubt it is part of the vocabulary of ordinary gallery-goers. 'Oversheet', on the other hand, appears as such in none of my dictionaries, not even the (albeit small) ones on paper, printing and bookbinding, so it is even less likely to be understood by English speakers.
Or should the translation be 'dumbed-down' so as to be understandable by Joe Bloggs? And should the curator be invited to adjust the French so that it is equally understandable by Jean Martin?

Note that Martin H. Manser's little dictionary of Printing and Publishing Terms defines 'overs' as 'Paper supplied beyond the ordered number to compensate for sheets spoilt during production', which lends weight to 'oversheet' for the companion term for an extra printed sheet compensating for damage.

Also '
Overs
Overruns, the quantity of items produced over the quantity that was originally ordered. Also referred to as any paper spoiled in the process of printing
http://printingtips.com/glossary-term/t--1653/overs.asp

Overrun – Number of pieces printed or paper made in excess of the quantity ordered.
Overs – Printed pieces in an overrun.
https://www.trumbullprinting.com/printterms/

Overs:
The quantity of unit production, for example, books and sheets, delivered to the customer above the net amount ordered, usually uncharged at a run-on rate; also allowance to cover wastage.
https://www.webmartuk.com/guides-whitepapers/glossary-printi...

Overs: Copies printed over the amount specified, needed for make-readies* and to compensate for spoilage
https://www.umsl.edu/printmail/printing-glossary.html

* "1) Make Ready is used as a phrasal verb to describe the initial set-up and adjustment process needed to prepare an offset printing press for a production run.
2) Make Ready is also used as a noun to signify the extra paper needed for printing test sheets while fine-tuning the press settings prior to the actual production run."
https://www.colorvisionprinting.com/blog/offset-printing-wha...