This is a link from the chain that stretched across the Hudson River to prevent British ships from sailing upriver. Two huge chains were constructed and stretched across the hudson, the more successful being the Great Chain, constructed from West Point in 1778 and used through 1782. This link is from that chain, measuring around 26.25″ x 9.5″. The links were forged in Orange County, New York. For source and more details, click here, and here.
Images from: http://www.cr.nps.gov/museum/exhibits/revwar/image_gal/morrimg/linkwestpointchain.html, http://en.wikipedia.org/wiki/File:West_Point_Fortifications.jpg