Saw this on eBay this morning. The cancel portion on the cover looks drawn. Compare the letters on the stamp to the letters on the cover, to me the font doesn't match. The giveaway is the difference in the R's.
I've seen a number of these over the years. There was a NYC dealer (Stampazine, IIRC) that I would buy 3c 1851's from back in the 1980's and many of them would be hinged to a piece of paper with the remainder of the cancel penciled in. I think, basically, the collector/dealer got a stamp (OFF piece) with an interesting cancel (partial cancel, due to the relatively small size of the stamp) and he/she felt it would add to the presentation by 'filling in the blanks'. No misrepresentation was intended.