A sterling silver ballpoint with a grain guilloche that carries no more than the 925 hallmark and the country marking Germany is in all likelihood an original from the 1980s. The fine grain guilloche was a common surface on the silver Solitaire pens at the time, alongside the better-known line guilloche with its continuous lengthwise lines.
The absence of an individual identification number gives no grounds for suspecting a fake; it is a sign of age. Montblanc began laser-etching a number onto its pens only around 1990. As to the country marking, both forms occur on pens of that period, “Germany” and “W. Germany”, and no mark of fakery can be inferred from that either.
A final judgement calls for sharp photographs of the clip, the inlaid star, the ring engraving and the mechanism, since the clip of all things gives copies away most reliably. If nothing there stands out and the hallmark and weight of the silver body are as they should be, there is nothing to argue against an original. Minor wear reduces the value far less than it does on collector's pieces in precious resin, because silver parts can generally be restored.