Although we talk about Mark II C5s starting in late 2004, they weren't given that name by Citroen. They were only known as 'face lift' models. In 2007 when Citroen wanted a new C5, they designed several styles which they called Projet 7, and by 2008, the new Mark II was into production.
But it seems whatever Citroen say now, to the man in the street, a MkII is anything from 2004 onwards, and an C5-X7 is a Mark 3.