Of course, with many pens there's no way at all to tell, and you're lucky to narrow it down to a particular decade.
My Parker Vacumatic is date-stamped 4th quarter 1945 for the barrel and 2nd quarter 1945 for the nib, but the cap and blind cap are obviously made from the same different piece of green pearl laminated celluloid from that of the barrel, and the long blind cap has a tassie and jewel which indicates it was made between 1937 and 1942.
My big Sheaffer's OS Lifetime balance is also an enigma: it was made in Canada, and has a WASP clip and an upturned nib; I have yet to find a reference that tells me when it was made, so I just think of it as WWII era and leave it at that.
My other Lifetime Balance is a slim model in marine green striated which puts it between 1937 and 1945, but that's as close as I can get.
For my modern pens, only my three UK-made Parkers have date codes.
Possibly the worst make to date is Sheaffer's. You have to rely on catalogues, and even when catalogues are availables, the "start" date of a certain production may be determined, but not the end date. A combination of ads, clips, sections, nib imprint, barrel imprint, lever pins, and other details must be used. It's fun!
As for other general sources I have the Lambrou (FP of USA and UK) and the Erano (FP past and present) books.
thats one reason why our advertisement gallery is such a wonderful research tool. I encourage everyone to add to it as often as possible. The bigger itgets, the more we all learn.
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Tu ne cede malis sed contra audentior ito ~Yield not to misfortunes, but advance all the more boldly against them