Sarastro is the name of a silversmith/jeweler in Pforzheim (the company remained active until 1986) known for the production of silver overlays present on many Montblanc, both for early safety and later models. Its overlays, decorated with engravings and high quality guilloché, are of absolute value. Sarastro is in fact considered the Montblanc "official" jeweler with regard to the decorations of precious metal, and a Montblanc with its coating is considered original production. These overlays are easily recognizable by the presence of a punch with a "S" in the marking of the metal titration, as in the photo at left.
-  Page on PenExchange that mentions "Sarastro".