It’s such a nice word, that it makes one even sadder about the passing of letterpress. We have no need to use the word flong any longer.
Flong was a papier-maché mould of a forme of type. If you knew you were going to have to reprint a book in the days of letterpress, you could either store the typeset pages locked up as they were in imposition order in formes — an expensive, weighty, and fragile operation — or you could make a mould of the type. This mould would be used in turn to the make a stereo plate. You’d probably store the flongs, only making stereos when you had to do the reprint: no point in risking the investment if the exact reprint isn’t actually going to happen; if the book fails to sell or the author comes in with lots of corrections. The flong was an exact reversed copy of the type — right-reading because the original type was of course wrong-reading so that when it (or the stereo made from the flong) was printed it would read right on the page.* One would make stereos on any letterpress job which was likely to reprint often, without alteration. Examples of such books would be The Holy Bible, and Godfrey and Siddons’ Four-Figure Mathematical Tables, a copy of which would be in every schoolboy or girl’s bag back then. (Both of them actually.)
Flong was invented in 1829 by a Lyon printer, Claude Genoux (doubtless his name would have been recorded in flong for many a UK French grammar textbook — “bijou, caillou, chou, genou, hibou, joujou, pou prennent x” as we used to be drilled. Wikipedia tells us that the derivation of the word is from flan, which does sound convincing since flong is how a Brit would have said that French word. But I wonder why M. Genoux referred to it as flan — it’s a bit of a conceptual stretch isn’t it?
* Hot metal type for letterpress printing had to read back-to-front, so it would print right way round.