Schweet! The last patent date is Sep 29, 1868 which is for a Job Davis shuttle which this model doesn't use. Google patents is a wonderful resource. I did some research on this machine previously and it falls between an 1869 Job Davis improvement on the vertical feed patent and an 1872 Jesse Hinds Back Hanger Improvement patent. My guesstimate would be 1869-1872 but a patent can be in use before it's actually patented (pending or filed for).
Jon