Nice!!
For some reason, the H & D Folsom Arms Co. used the name "American Gun Co." on the guns they made at their Crescent Fire Arms Co. factory in Norwich, Conn. that they marketed, and that they sold to others that weren't ordered with a "trade brand" from the late 1890s until after WW-I. We do find a few higher grade guns from this period that are marked "Folsom Arms Co. New York" --
Engraved H&D Folsom 379074506.jpg
Early on these little hammer doubles were offered in 28-gauge and .44XL.
Catalog No. 11 Small Bore Double Barrel Guns.jpeg
Along about 1915 they began chambering the No. 44 to take the newly introduced in North America 2 1/2 inch .410-bore shell.
Catalog No. 18 Small Bore Double Barrel Guns.jpg
By about 1920 they began using the Crescent Fire Arms Co. name again.
Catalog No. 23.jpeg