As there are no markings, it is difficult to say with certainty what metal this pen is made of, but it does not feel like it is made of sterling silver, unlike the 75 production model with a similar pattern, known as the Damier 75 which were available in sterling silver and vermeil.
Also note the end tassies don't have the familiar facets, and the the faceted ring on the open cap end is omitted. Here is a look at the ends with the round onyx inset that also shows the Damier-like pattern.
