No. The International Service Certificate does say that the “925” hallmark and the Montblanc maker's mark together guarantee the authenticity of the precious material, but both marks are found only on newer pens. Older Solitaire models, and historic pieces with silver overlay all the more so, carry the fineness mark alone. The absence of the additional house mark is therefore not a sign of a fake but a sign of age.
Much the same applies to the clip: older Montblanc pens carry only the country of origin, “Germany”, with no identification number, the serial number having been introduced later. A worn twist mechanism, too, is a matter of wear and not of authenticity; it can be put right by the factory service or a specialist workshop.
The surest way to a dependable verdict on authenticity and servicing needs is to take the pen into a Montblanc boutique. Individual hallmarks on their own seldom allow a firm conclusion without the pen in one's hand.