A missing silver mark points not to a fake but to the material used. Vermeil means gold-plated silver, and only those versions carry the 925 hallmark. Solitaire models were, however, also made in gold-plated brass, for which no hallmark is prescribed. If there is no stamp on the plain engraving panel, what you have is not a vermeil but a gold-plated Solitaire in brass – a regular production model, not an inferior copy. The designation fine-grain describes only the guilloché pattern on the body, not the base material.
The wording “W. Germany” dates the piece to before 1990. Origin stickers still surviving on barrel or cap are a matter of taste: if you want to use the pen, adhesive residue comes off quite safely with a little lighter fuel on a soft cloth. If the piece is destined for the collection, the original sticker is more of a plus and should be left where it is.