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...
"Far to the general now, and without great final hope of victory, he has nothing to lose. And it must above all have rabies. For the moment, it dominates this special."