7/16x20 definitely, theres a few types of bolt used on the belts, have a rummage in a breakers 
beware that newer stuff kike the C2 went metric, bugger if I know why they were imperial to start with on a french car, but to change after decades makes it even more baffling
balls grinding always makes me snigger

beware that newer stuff kike the C2 went metric, bugger if I know why they were imperial to start with on a french car, but to change after decades makes it even more baffling

balls grinding always makes me snigger
need a part number? http://public.servicebox.peugeot.com/ and http://service.citroen.com/ will sort you out.