It looks like it left the factory in about 1938 according to this chart:
http://doublegunshop.com/dgsnos1.htm. Scroll down to the 20-ga. Sterlingworths.
Utica-built Sterlingworths typically have a lower perceived value than those that were Philly-built. It has been reported that the build quality also went downhill in the latter years of production and some of the refinement (a more bulbous forend, funky looking trigger guard, no grip cap, etc.) usually associated with Philly guns was also lacking. Having said this, they have 2 3/4" chambers and likely have higher, more modern dimensions than the earlier guns have. Without good pics it's impossible to say with any degree of certainty what the value is, but you rarely find a 20-ga. Sterlingworth in decent condition offered for much less than $1200 anymore.