J.W. Waterhouse

If any members of this site can offer an opinion or some expertise, it would be greatly appreciated. Recently while in San Francisco, I purchased what I think might be a vintage lithograph of "Pandora". Image size is approximately 13 1/2" x 24". It was in a gesso "bronzed" pie crust type frame with the old wavy, seedy glass. The torn liner on the back did not look original but was not recent either. 
Once back home, I disassembled the piece to clean the inside of the glass and the frame liner and wipe the print off as they were a little sooty. The print itself was printed directly on a type of old art board that was about 1/4" in thickness rather than any type of paper. It was definately old as it had the musty smell that sometimes comes with being in an attic. It was held into the frame liner by small nails that looked like they were never removed as the nails left an imprint in the back of the board. The print itself which has some very slight fading but otherwise in pretty good condition looks original to the frame. I know that vintage prints/litho's are out there for Pandora but have not had the experience of ever seeing one. Were they printed on paper or this type of "board" that I've found? Nor do I know at what time periods they might have been produced.
I've recently come to appreciate the work of Waterhouse, Leighton and the likes so naturally I'm quite curious to get some info on my find. Based on my description, is this ringing a bell with anyone? Can someone offer any validation that this might be a vintage Pandora?
Won't be disappointed if it's not because the wife and I love it anyway but would like to know.
Thanks,
Dan