TOP DESCAR GAS MP3

I recently came across a lithograph or print at a local antique market, I recognized the artist : Peter Paul Rubens. This print or litho is of a young woman with her head turned to left. This is a study drawing for his tryptic altar piece painting "The Miracle of St. Ildefonzo" in the sevententh century. It is signed "P P Rubens on lower right side. I have tried to find any fine art print companies who might have had access to print this but have yet to come across any. Also it took me two weeks just to find out what the image was, so I am guessing it was not mass printed. Any idea what a print or lithograph like this might be worth?
I looked a little closer near the signaure, it looks like it says 6/?
Im not sure what bottom number is but top is definately a "6". I am guessing this is the print number, but I could be wrong.

here is image of signature with some numbers below them