A writing instrument like that is a Meisterstück Solitaire Doué Vermeil Classique. The addition Doué denotes the combination of a cap in precious metal with a body in black precious resin. Vermeil means gilded sterling silver, which is why the cap carries the silver stamp 925 despite its golden appearance; the oval mark struck beside it is usually a maker's hallmark. The lengthwise fluting with a plain panel for a monogram engraving is the cap pattern typical of this range.
A burgundy piston filler with a striped ink window and the engraving Montblanc Meisterstück running round the cap ring, by contrast, belongs to the Meisterstück Le Grand range. Each of the two may well be genuine in itself, but together they do not make a set, since they belong to different model ranges; they were never supplied as a set. A missing serial number is no argument against authenticity on older pieces, as Montblanc only later began marking every writing instrument with an ident number.