I believe the largest collection of Escher artwork is the Vorpal Gallery in San Francisco. The owner of the gallery used to be the distributor for Escher, and after his death, re-purchased many of Escher's works -- especially since as a dealer he knew who he sold the works to. I've been in the back room and have held some of those original works in my hands. :-D Call them and find out what their opinion is.

Calvin