It's a "gonfleur
" (literally "inflator") - a canister of compressed gas. I think I read that nitrogen was more commonly used in those days, but don't quote me on that.
They were commonly seen on Tour bikes in the late forties and through the fifties when mechanical support was restricted, and riders often had to deal with their own punctures - replacing the punctured tubular with the spare they carried wrapped around their shoulders, and inflating it quickly with the gonfleur
to get back in the race.
Check out the scans from newspapers of the period on Aldo Ross's Wool Jersey gallery:
http://www.wooljersey.com/gallery/v/ald ... x.jpg.html
It's interesting to follow the changing fashions of bottle, pump, and gonfleur
Calmes dans le demi-jour
Que les branches hautes font,
Pénétrons bien notre amour
De ce silence profond.