Here we have the celluloid 644 in grey stripe colour with its beautiful rolled gold cap from the mid-1950s. According to an old Dutch catalogue, there was also a silver cap option for about 50% cheaper. The 6xx is part of the Meisterstuck series, and it has the telescopic piston system. Its design is very distinct. Another two series - the 7xx and 27x (Danish) - look similar. There is also an "N" version which I believe has the winged nib like the 25x series. The 64x also comes in striated green and black which are beautiful in their own right.
I have two 644s - one was made for the German market and has "Meisterstück" engraved on its cap, while the other was made for the export market of India (see review --> link).
This pen is from the collection of Albert Domenech in Barcelona, Spain. He is an expert in antique artefacts, jewellery, watches and pens. You can visit his store here --> barcelonantiques.es
A beautiful silver striated 644 belonging to Albert Domenech