^^^ What Madcrate said. The stuff is like a very old, weather-beaten tire; it is very hard, has a glossy exterior appearance, and is not pliable like most modern rubber products.
I suspect that most people under the age of 40 have little exposure to this material; it seems to have rarely been used on postwar consumer products, probably because it is more expensive to produce than similar plastics. FWIW handles of old tools and some parts of old toys were made of similar hard rubber.
OTOH if the interior of the grips are hollow, they are injected-molded plastic and are definitely NOT original. FWIW I have seen somewhat old (probably 1970s vintage) injection-molded reproduction S&W stocks at gun shows before, albeit not for vintage I frames.
"Smokey, this is not 'Nam. This is bowling. There are rules... MARK IT ZERO!!" - Walter Sobchak