Hey, this is NOLA_ken's girlfriend. I know a little bit about painting and art history, and you should bear in mind that the number on the back is not necessarily a date. It could be an opus number, or some kind of stock number from a store/gallery that sold it before. Also, is the fourth picture the back of the canvas(under the frame backing?) What kind of back does it have? (cardboard, wood, etc) It doesn't look like normal canvas to me, or something that would have been available in the 18th century. It looks weirdly shiny, like some kind of more modern thick synthetic fabric. Like woodstock said, the little R on the left is odd too, most artists would just finish the signature over there instead of trying again on the other side. You should also keep in mind that gesso was not strictly an 18th century thing. It is still used by painters now and you can buy it at Michaels.