Winslow Homer original?

I was at an older friends house the other evening who showed me this newly framed work he owns, "Waterfall in the Adirondacks":


he's had the piece for many years. he bought it at a museum that had a Homer exhibit at the time. as it turns out, it may be an original - from which the litho's were made. I forget what the term for it is; not a "master" but some sort of proof or something is stamped on the back. nice watercolor.

maybe something to take on Antique Roadshow next time they're in town?