Those grips appear to be injection molded - the round mark at the bottom looks like an ejector pin mark. I can't find anyone today who is making those grips, although other S&W grips are being reproduced.
Hard rubber, or gutta percha, is a natural product like rubber, but from a different plant. It was used quite a bit prior to the discovery of the vulcanization process for rubber because unvulcanized rubber is brittle, where gutta percha is flexible until it ages. Its use then diminished, but continued until the discovery of man-made plastics.