The Solitaire Doué Signum belongs to the Meisterstück Solitaire family and was probably launched in 2003; it has since been dropped from the range. Its defining feature is a platinum-plated surface over a body of brass.
Since this is a plating and not solid precious metal, heavy use brings the yellowish brass through at the edges, at the cap lip and on the clip. A pen showing yellow metal over a large area is therefore not necessarily an unrecorded special version, still less a fake; as a rule it is simply well worn, and the plating is occasionally rubbed off on purpose. Replating can normally only be carried out by the manufacturer's service department or a specialist workshop, and is worth the outlay only on a pen that is otherwise in good condition.
It is also worth knowing that the line was produced in corporate-gift versions, with a company logo worked into the cap crown in place of the Montblanc star. Such pens are genuine, but the collector market rates them below the standard version. Whether gold-plated variants of them were also made remains unresolved.