2.5" Colt Diamondbacks sell very easily and prices are generally higher than the 4" variation. The snubbies are popular and desireable. You can find replacement service stocks for the Diamondback. But if I were the seller, I would sell it as-is and let the buyer find the replacement stocks.
Colt began production with the 2.5" and 4" Diamondbacks in 1966 and later produced them in a 6" barrel beginning in the late 1970's (78-79) until production stopped in 1986.
There is nothing wrong with those grips. They just aren't original. I suspect that they are Jay Scott grips that Colt pushed back in the day.