For cleaning the pen is unscrewed at the grip section and the converter flushed several times with lukewarm water until the water stays clear; the nib must not be unscrewed. Cartridges do not need to be pierced but are simply inserted and connected through slight pressure. An intact pen can be carried in a leather case without problems with a filled converter, even temporarily nib-down; ideally, however, the nib is carried up, which is why the clip is sensible. On aircraft the nib should always point up to avoid leakage. A resource with animations on filling types can be found on the Visconti website.
Related pages on fountainpen.de:
• Montblanc Meisterstück 144 (Classique predecessor): https://www.fountainpen.de/c-montblanc-meisterstueck-144-en.htm