Lacking a detailed description, including ALL markings, gauge, type of action, length of barrels, etc., most likely turn of the century.
Christopher H. Sterling was born in 1943.
The value of a HJ Sterling Belgian-made double barrel will depend greatly on its condition. A gun like this can usually get between $75 and $250.
Blue Book of Gun Values
No published sn data I know of.
Almost certainly not - likely less than $100 if it is a useable shooter.
25-100 USD or so. E-gunparts.com, want ads, gun shops, gun shows.
I can't help you with the age, however a source for single shot shotgun forestocks can usually be obtained at http://www.e-gunparts.com/ Good luck The Victor Ejector was manufactured by Crescent Fire Arms, 1893-1932. American Gun Co was a house brand for H&D Folsom, a wholesale/retail sporting goods company of New York.
50-90 yrs
Try a good gun shop. H&R makes a fine single shot 20 gauge.
I would not do it if it is choked more than Modified.
Blue Book of Gun Values.
Might be a code for a date, inspector, lot #, etc..