I recently splashed out nearly €23 on Citroen's special tool 1677-T, used for the set-screw heads that I think only Citroën ever used:



In my case it was so I could undo the gearbox from the engine of a 1960s HY-78, but you'll see from Der Franzose's catalogue that they were also apparently used on DSs and 2CVs - presumably the older models.

The picture also shows an actual bolt and socket.

I am curious to know what advantage this shape of head actually has? It obviously didn't catch on widely.

My other question, though, is about a tag I found rivetted to the top of the H-van gearbox:



There's nothing similar on my own H-van gearbox, and I wonder if perhaps it was put there when the gearbox was rebuilt. If so (or even if not!), could anyone suggest what the numbers might mean?

My own guess was that the number on the right might have been the odometer reading when the gearbox was installed (as this was 80000 or so when the van was eventually taken out of circulation). But the other one - 103?

I have seen this type of plates on several 2cv boxes too. The older ones mostly. I think they are serialnumbers from production.