Ship-shape and Bristol-fashion

I was away in England for a week, not speaking, writing or reading French (despite all intentions), hence the silence here.

Back in Lyon, I found in my mailbox yesterday an unsolicited copy of the newspaper Le Figaro--a promotional freebie, I assume. Always glad to have new reading matter, I've been exploring its pages, and today found an article in the culture section about the quest for undiscovered manuscripts, musical and literary. The piece, by Thierry Hillériteau, mentions Nabokov's unfinished novel, The Original of Laura, famously published in 2009 (2010 in France) against the author's wish, and a sidebar gives more detail.
"Le livre....reproduit fidèlement le manuscrit avec ses bristols qui servaient de brouillons à l'auteur." 
"The book...faithfully reproduces the manuscript with its bristols which served as the author's rough drafts."  Les bristols? As a Nabokov acolyte, and owner of a copy of the book, I'd have realised, even without the photograph in the newspaper, that bristols must refer to the index cards on which Nabokov composed his novels. But why is that the French word for them?

Investigation in my French-English and French etymological dictionaries reveals that this comes from Bristol board, still used in English for a special type of fine cardboard, originally made in Bristol. In French the abbreviation bristol means "Bristol board;" le bristol also meant "visiting card" in the days when these had an important social function. Neither of the dictionaries mentions that le bristol can also be "index card," but that's obviously the case here.

Haunter of stationery shops though I am, I haven't noticed whether index cards are actually sold as "les bristols", but I'll be sure to look next time I'm browsing through (I almost forgot the "through"--lived in America too long!) a tempting array of papers and blank notebooks and such.

Of course I can't end without a glance at what some readers (you know who you are) may have had in mind all along, the very different meaning of "bristols" in cockney rhyming slang, from Bristol City=titty. This usage seems to have emerged in the early sixties. Why Bristol, and not some other city?  Various reasons have been suggested: the city's football team is "Bristol City" so the phrase was already in common usage; there's an obvious echo between Bristol and breast; Bristol was famous in the 19th century for large-breasted prostitutes. I list these with no idea which, if any, is more correct or likely.