Last Updated: Aug 28, 2017 Views: 54
We have both books you mentioned in our Library collection, and I took at look at the piece you are interested in. Reverse paintings on glass: The Ryser collection, provides the following information about the piece: Catalog no. 70, "Portrait of a Lady," China, about 1770. Verzauberte Bilder has a section on "Chinese reverse glass paintings for the European market" - pg 223-227 (in German) - that may provide some contextual information.
I found that the Ryser collection is currently held by the Vitrocentre Romont, and it looks as though the "Portrait of a Lady" is currently on display in the Vitromusée Romont. You may wish to contact them to see if they have more information on the piece.