The CI definitey means contributions indirectes, see at the link redwine gave above.
I quote with google translates help:
...in 1942, the plaque was printed on cardboard as a facsimile of the metal plate. The price passed this year to 25 francs. This rate was visible on the plaque for the first and last time. In the municipalities where this cardboard plate was no longer available, the local authorities issued a car registration with the mention that it was a bicycle. These last pieces are rare, even if hundreds of thousands have probably been issued.
The following year, in 1943, the bicycle plate was issued in the form of a stamp that could be glued on a support or a piece of cardboard. These stamps are extremely rare and have often been kept by philatelists. It is not uncommon for plates to be found in the handlebars of old bicycles. Some put it there to be able to produce it in case of control.
From 1949, the stamp will be replaced by a nominative form. The name of "bike plate" will be kept. It was sold in tobacco shops and will be issued in five different colors, after came the vignette. The last form will be issued for the year 1959. By a ministerial decree of December 30, 1958, the French tax on velocipedes will be permanently abandoned...
