A missing visible carat marking is not necessarily an authenticity feature: on various transitional models, e.g. the 144 from the 1960s, the carat hallmark sits very deep on the nib body and disappears two-thirds into the section, so that without disassembly it is barely visible. Nibs from the 1980s also deviate in engraving and stamping from current nibs. Collectors strongly advise against pulling the nib out yourself, since the special slots on the body require a suitable tool and knowledge of the turning direction, and Montblanc uses a special sealant when reinstalling; a self-damaged nib seat can only be repaired in Hamburg. Instead, comparing the whole pen with the model overviews on fountainpen.de and, if necessary, an enquiry with Montblanc or an authorised specialist dealer is recommended.
Related pages on fountainpen.de:
• Montblanc Meisterstück 146 (Le Grand): https://www.fountainpen.de/c-montblanc-meisterstueck-146-en.htm