These are tags for bicycle and showed that a special tax, called "contributo manutenzione stradale" (tax for road maintenance) had been paid. As you can see from the pictures, they're oblong to be attached to the bike as a ring at the steering. I don't know too much about them, but I was told that they were used until the Second World War: anyway bicycles don't pay any tax today.
You can click on the images to enlarge them.
Many thanks to Louis Fierens, to Roberto Pola and to Alessandro De Zuani for the pictures.