I'm not sure what you want to know. A "shotgun" is a long-barreled firearm that fires many small pellets instead of one solid bullet; "20 gauge" is the diameter of the barrel; and "single barrel" simply means it has only one. Do you perhaps want to know what it would be used for? It would be used to hunt small animals and birds or to protect the henhouse from preditors. Or do you want to know what it is worth? There are a few models made by Winchester, Ithaca, or other top-line manufacturers that have a collector value but the majority are utility guns worth from $25 for a broken piece of rust to $150 for one in excellent condition.
50-75
50-100 USD
245
Try e-gunparts.com
The 12 gage has a bigger barrel then a youth 20 gage shotgun.
Maybe 150 or so
Most likely turn of the century
Single shot, $35-$75. Double barrel, $100-$200.
Typical Crescent single-barrel shotgun. I have a good supply of parts. sales@countrygunsmith.net
Go to google and enter "20 gauge double barrel shotgun" then click the (images) button at the top of your screen. Then hit (search images).
The Sears Ranger 105-20 16 gauge shotgun came out in the 1930s. The Sears Ranger shotgun has a 26-inch barrel.
Utility grade shotgun, 50-100 USD or so