Collectors do not classify such pieces as prototypes. More likely is either a subsequently inserted spare part (nibs frequently broke and were replaced by other contemporary nibs like an Astoria) or an assembled pen with barrel and cap from different models, since Montblanc never delivered writing instruments with mixed brand markings. A combination of round barrel form and octagonal cap also speaks against factory originality. A missing star and unknown model variants are, however, no unambiguous counterfeit indication, since before the Second World War many models existed and Montblanc's own pattern department was destroyed in the war. A reliable assessment is only possible through inspection in person.
Related pages on fountainpen.de:
• Astoria — brand overview (fountainpen.de): fountainpen.de/astoria-en.htm