I thought it looked like a model 66 also, but for some reason- one of the Walther pages (might have been the "Unauthorized Walther Page" said it was based off the 686)-interesting. I also, would not pay much more than whatever S&W model the R99 is based off of (Maybe $40 more).
Besides the grips, and different markings
marked "Walther La Chasse" somewhere on the frame.
I don't think much else is different