Special M8x1.25 nut with 12mm hex for carb base to manifold on 2cv, Dyane, Ami etc with twin-choke carb (+ Renault 4 etc).
Citroen used M8 nuts with 12mm across flats, instead of the standard 13mm, so a 12mm spanner can be used, as there is insufficient room around the mounting studs for a standard M8 nut and a 13mm spanner.
There are 4 per twin-choke carb but as you may be just replacing a missing one, we are selling them individually.