that was when I was a little kid in the late 1950's. My father was a stamp dealer and got a lot of them in. Various dealers offered "subscription services" for them in those days. Somewhat like various chip collectors offer to run to their local casinos and pick up whatever new chip issues come out, for which we are all grateful.

But first day covers seem to have gone passé many years ago. Today most are worth only the value of the cancelled stamp(s) on them.

Incidentally, during the years 1940-45, a larger-than-usual number of FDC's were obtained and hoarded away. Reason: it was thought they would be eagerly bought by our servicemen once the War was over, so that their collections would become up-to-date. This didn't happen, either our stamp collecting soldiers perished in the war, or their interests changed once they returned stateside.

Still, your collection may have some value, perhaps a new crop of stamp collectors will go ga-ga over them.