My guess is that your grips are made of a phenolic plastic called Catalin. It was very popular back in the 1930s and 1940s, and was one of the very few plastics made commercially at that time. It was used for jewelry, appliances, radio cases, kitchenware handles, electrical components, etc., and of course gun grips. It was available in many colors. Anything made of Catalin from that time period is highly collectible. You might want to search on Catalin to find out more about it. It has a distinctive smell.
Do you still have that M&P snubby? If it is pre-WWII, it is quite rare and fairly valuable also. Can you provide its serial number?