Dating a 149 rests on several small details that changed over the decades. One pointer is the hallmark on the nib: the form “18 C” is regarded as characteristic of the 1960s, while other ways of stating the gold content belong to other production periods. A second is the feed, which on examples from the 1950s and 1960s has two fine lines down the centre. The gold ring at the end of the barrel is also informative: if it matches the 1950s pattern, that argues for early production.
Taken on its own, none of these features proves anything, since the nib and feed may have been replaced during a repair. Only when several pieces of evidence agree does the dating become solid. Pinning a pen of this period down to a single year is not possible in any case; the usual and honest approach is to narrow it to a decade or to a production window of a few years. Direct comparison with a securely dated example is very helpful, because differences in the rings, the feed and the nib are far easier to see side by side than in photographs.