A few years ago on eBay I saw a road book holder that was actuated by a pushbike type hand brake set up (advancing only as far as I could tell), it still had the original box and I seem to recall the seller saying it came from the 80's.
Unfortunately I didn't win it so I have no more info, and it wasn't what you asked anyway, but I was just curious about the set up and wondered if anyone remembers them.
Oh and I mention it because I have a feeling you want to build a period correct Rallye bike, just giving you options...
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