Indeed, the steering lock was inserted through the back (or front) of the head tube (a little tube brazed on), and immobilized the steerer tube inside, usually at an angle. So you could roll the bike, but not in a straight line.
P. 9, drawing 7 has an example on an Herse, also the centerfold Rebour drawing on p. 13/14 (which also shows the "eclairage sans fil" - the transmission of electricity from the rear mounted dynamo to the front light without exposed wires by having a carbon brush inside the head tube running on an insulated brass ring on the steerer tube).
My mom's Peugeot folding (take-apart, in fact) bike with small wheels in the 1970s had that feature, and I remember it worked well, but didn't last.
Another lock popular when I was a child in Germany was a bar that swung across the spokes on the rear wheel. The key stayed in the lock when it was open, so you wouldn't forget it. Unfortunately, those you could pry open with a screw driver. Lightweight and locks don't seem to go together well!
These days, with rides to coffee shops popular, it might be a good idea to think about an elegant, lightweight lock, not to deter a professional bike thief with tons of tools and time, but to prevent someone from hopping on while you are watching from the distance...
Question to all: p. 18 shows a "Neiman" lock on a Pitard - how does that work? Looks like a more or less standard wingnut to me, but somehow it must immobilize the bike... I have no idea!
Why the locks on superlight technical trials bikes? Because you were awarded credit points that more than offset the added weight if you had one.
Jan Heine, Seattle
Vintage Bicycle Quarterly
The Rebour drawings are such that being isolated on a plane for 3 hours was the perfect time to study them in detail. I had never seen a "steering column lock". I want to say page 22, but again, typing from memory. While I couldn't be sure, it looked like an insert thru the backside of the headtube into the steering column to prevent the fork from being turned. Jan Heine or anyone else, comments? Another nice job on your publication Jan. Lou Deeter, Orlando FL