Your Stevens model 240 was made from 1940-1945.
Currently they are bringing between $200 - $400 depending on the condition.
The Stevens 240 is an over-under .410 with double hammers, double triggers and a Tenite (plastic) stock and forearm, modified and full choke. It was made from 1940 to about 1948. Values in 2005 seem to range from the mid-$300s to upwards of $500 for good to excellent specimens. Gunshow prices have really been going up for these guns.
The patent date you are looking at does not indicate a year of manufacture. Stevens made many different models of double-barrel shotgun. Which model do you have? sales@countrygunsmith.net
Serial numbers were not required before 1968. If it has one, check under the forearm, on the barrel and on the action.
Depending on which entry in the Blue Book you believe, it was either produced from 1912 to 1931 or from 1926 to 1935.
I can say that the Stevens model 311 side by side shotgun was discontinued in 1989.I would go to the Savage arms web site,they may be able to help you find out when the manufacture of your shotgun was started.
I know that if it has a plastic (tenite) forestock it also had a plastic buttstock. Manufactured probably in the 1940's or 50's.
The cast of Body Double with Brinke Stevens - 2013 includes: Brinke Stevens as herself
L Double Outer Forearm Block
it's probably a model 5100. the tenite (plastic stock and forend) as it was called was likely made sometime around wwII, because of a shortage of wood built gunstocks and forends
$20-50.
Muriel M. Stevens has written: 'Double thread'