"Knickerbocker" was a Model of shotgun sold by the H.D. Folsom Company.....and the American Gun Co. was one of the hundreds of brand names used the Crescent Firearms Co. of Norwich, CT.
Crescent was the largest maker of "Hardware Store Shotguns" from the 1890's until about 1930. These were made for and sold by a variety of hardware companies, mail order stores, retailers and distributors. Nearly 150 names are known or thought to have been used on shotguns produced by Crescent.
Most of these Crescents sold for about $10 to $12 around the turn of the century, which placed them in the low-to-moderate price range.
Although not an expensive gun, they were well made and a good value for the money.
Values range from $100 to $200 depending on condition.
A Note of Caution ....depending on when your gun was made, it may or may not be safe to use with modern smokeless powder shotgun shells. Many shotguns manufactured before about 1920 were made for blackpowder and should not be used with modern ammunition. If in doubt, take to a qualified gunsmith and get his opinion.
BTW....I've got a soft spot in my heart for these old Crescents. For many years I lived just down the road (about 12 miles) from where they were made. Crescents are sort the Rodney Dangerfield ("I don't get no respect") of shotguns....but don't ever let anybody tell you they were junk.
Every time I go to a gunshow, I make it a point to look and find old Crescents (I have a list of about 180 names under which they were manufactured).....and I've seen a lot of 'em in various conditions, but I've never seen one that wasn't a very well made gun.