Single barrel shotgun on frame left side says Stevens model 9400 savage arms westfield mass USA on barrel proof tested 20 gauge 2.75 and 3 inch chamber in small circle 39S when was your gun made?

The Stevens Model 9400 shotgun was produced by Savage Arms, with production of this model occurring primarily between the late 1960s and early 1980s. The specific markings on your shotgun, including the "39S," suggest it may have been manufactured around 1972, as this code corresponds to a production date. However, for an exact date, you may need to consult Savage Arms' records or a knowledgeable firearms historian.