Re: Sterlingworth/Fox 20 gauge double barrel - worth?
Welcome to the forum.
You're pretty much in the ball park as to when your 20 gauge Fox Sterlingworth was made: the late 1920's.
It looks like the receiver has been either, or both, polished or plated. The original finish on the receiver would have been case colored which is a mixture of blues, greens, etc and with use and wear, would take on a subtle blueish gray color to just a dull to shiny finish. Your gun has a high polish looking finish and to me looks like it may have been plated.
That takes it out of the collector category and makes it a nice shooter. If the gun is sound and the rest of the gun is original, my guess as to its' value would be in the $800 to maybe as high as $1200 if you find the right buyer. That's the best estimate I can come up with from your pictures.
Hope this helps and hopefully, you'll get some more evaluations.