I noticed you called it a skeleton key. It is likely a lever lock key blank. Could be a warded key blank but doubtful being a 1865 patent. Yes it could be from prior to receiving the patent but much more likely after. Once a patent is no longer in effect many companies would just not stamp it.
Skeleton keys are keys that have been reduced in material "skeletonized" in the hopes of opening a warded and in some cases very simple lever locks. Warded locks and keys go back hundreds of years long before lever locks were used. Sometimes complex warding could be overcome by making skeleton key removing everything except what was necessary to get in and throw the bolt.