I'm about 99% sure they go on a double action Colt. The pic I saw on another forum looked far too big to fit a D frame (detective special, cobra, diamondback etc.) Could be an I/E frame (Python , Official Police, Officers Model etc.) but more likely the large frame New Service.
As to Elephant Ivory, I wouldn't want to say for certain unless they were in my hand. The grain really isn't visible enough IMO , in the pic to say for certain. The grain I can see, seems to be running in the opposite direction that I would expect to see on elephant ivory grips. It seems pretty clear it's some kind of natural material though. Bone, some other kind of ivory etc.