Ralph, it could be a coating, since hard rubber is molded in liquid form and not normally prone to peeling . . . BUT long term submersion in water with a wetting agent (detergent) can cause rapid deterioration. A quick quote from a definition article on Ebonite, an early hard rubber containing sulfur:

Exposure to moisture bonds water with free sulfur on the surface creating sulfates and sulfuric acid at the surface that are very hydroscopic. The sulfates form a film with favorable wettability characteristics on the surface. These aging processes will gradually discolor the surface grayish green to brown and if water or moisture is constantly available, it can cause rapid deterioration . . .

Hard rubber, like Ebonite and the type used by S&W contain up to 50% sulfur, which can cause the formation of sulfates in water. I believe that is just what may have happened and you have de-bonded the surface of the stocks. Unfortunately, not sure there is any solution to this problem. Maybe when all the moisture is dried, the stocks will once again be serviceable?? At any rate, I would not soak hard rubber in water more than a simple wash and dry in the future, even if there is the slightest chance it could deteriorate the surface of the stocks.

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Gary
SWCA 2515