wt1,
My Scott US Specialized is a 1983 edition so I can't tell you much from it other than that it listed six color variations for Scott 65: rose, bright rose, dull red, rose red, brown red, and pale brown red.
I checked the National Postal Museum Collection at the Arago site (
http://arago.si.edu) and it reflects that the former Scott56 is an essay with the Scott Catalogue designations of 65E##_.
This image from the collection is described as 65E15h (red brown) -- there are at least a half-dozen other images of the essays at the site.

As you can see, the ornament on the corners of the essay differs from that on #'s 64, 65, or 66 -- so I think it is safe to rule out it being a former #56, despite the penciling on the back.
Given the number of color variations for this 3c Washington, your only option to identify the stamp may be to have it expertised.
Steve