@ardgedee I assume it's some kind of flying buttress-like support for that huge span of glass in the rear. I'll bet this was some kind of camera vehicle for the front of the race.
It’s a Type H Citroen (https://en.wikipedia.org/wiki...) converted as an advertising vehicle for “la Fédération des producteurs de produits alimentaires dérivés du lait.” It sounds like the ribcage would have originally been covered with an advertising livery. According to the auction site, it’s probably the work of a Rouen coachbuilder known as “Le Bastard”, and was restored for a 2001 comedy, which explains why it’s in decent condition.
https://cars.bonhams.com/auction...
https://en.wikipedia.org/wiki...