GA-Dave,
Value will depend on the following:
-Has it been modified in any way (refinished, recoil pad, barrel shorter than 20”, etc)?
-Do all the serial numbers match (barrel, receiver/trigger guard, buttstock)?
-What is the graded condition of finish and wood, including the fit of the butt plate?
It will range from $300 to $2000 depending on the answers to those questions. Modified guns with recoil pads and poor condition mismatched guns can be had for $500-$800. All original, matching numbers guns in good condition usually start around $1000 and go up.
The serial number will be between around 39000 and 54000 and may have checkered or smooth wood.
James,
I'm a collector and researcher of Stevens 520/620 shotguns.