To add a bit more interesting and totally useless information. Your gun is technically what is known or called a "Trade Brand Name" shotgun. That is a shotgun made by a major maker (and between 1880 and 1940) for and was sold by a wholesale sporting goods dealer, a retail chain store or an independent seller (your local hardware store) who chose the name to go on the gun. The names KNICKERBOCKER and AMERICAN GUN CO were used by the H & D Folsom Company of New York City, a large wholesale sporting goods dealer, retail seller and a jobber who incidentally owned Crescent Fire Arms Company. Folsom bought Crescent to supply them with a lot of inexpensive shotguns fast which Crescent did. Folsom claimed in their catalogs that they made the guns. Not quite correct. Folsom located in New York City had no manufacturing facilities. All the guns were made in the Crescent factory located in Norwich,CT. To pin down the date your gun was made a little closer. It may have been said that all Crescent's records were destroyed in 1943 but were reestablished or reconstructed by Mr. Joseph T. Vorisek during his research for his book "The Breech Loading Shotgun In America 1865 to 1940." Joe took the number of a particular model of gun made and divided that by the years it was made and came up with an average number of guns made per year. That may not be too accurate but it's all we have to refer to.
His files show that your hammerless double barrel was made in 1909 as said. The serial numbers for that year start out with serial number 140,00 and end with serial number 168,00. Subtract those numbers and it comes to a total of 28,000 shotguns of this type were made in 1909. Divide 28,000 by 12 and that shows an average of 2,333 shotguns per month were made. Doing the math indicates that your gun was made about July 1909. Of course we all know that a shotgun is not made in a day or even a week. A word of caution at this point. Your gun was made using the technology and metallurgy of the times and designed for the ammunition in use at the time which was 2 1/2 inch shotshells loaded with either black powder or maybe, just maybe early low pressure smokeless powder and lead shot. It was not designed for more modern and longer . 2 9/16 or 2 3/4 inch shells loaded with higher pressure smokeless powder and certainly not 3 inch magnuums loaded with high pressure smokeless powder, steel shot or solid slugs. Since none of us here on the forum can see your gun to determine its condition, we must recommend that you do not attempt to shoot it. These were well made guns and can hold up with shooting shells loaded with smokeless powder if in good condition, we can't recommend it. If you want to shoot the gun, take it to a good competent gunsmith for a check out first and follow his advice and then only with proper ammunition. Value? These were inexpensive guns even when new, selling for between $15 and $25. They haven't appreciated that much over the passing years. Current value will depend on the guns condition, the amount of original finish remaining on the metal and wood as well as the mechanical condition. A prime condition example that appears to have come out of the factory yesterday afternoon (rare to very rare as these guns were used hard and received little care and maintenance) might fetch as much as $150 at auction while a rusty and pitted metal, rotten and broken wood and missing parts piece of junk fit only for parts salvage or as a whisky still stirring stick bring as little as $10 if it sells at all.