The 644 was built from 1952 to 1959. During that period the winged nib — a nib with wings splayed out at the sides that rest on the section — replaced the open nib used until then. Collectors date the first pens with winged nibs to the autumn of 1955, though the changeover was gradual, since parts and pens already made were sold off first. Both nib forms can therefore be found on bodies bearing the same model number, without either pen being wrong.
The difference has nothing to do with special export versions; it is purely a matter of date. For dating a 644 the nib form is thus a useful but only rough guide: an open nib points to the first half of the production run, a winged nib to the years from about 1955/56 onwards. A pen can be placed more precisely if further features are brought in as well — the cap ring, the shape of the clip and the design of the ink window.