Welcome to the forums from the Wiregrass! Model 15 Combat Masterpiece from 1977/78. Like 45ACP230 says, open the cylinder and read the model number from inside the yoke. It is probably stamped MOD 15-4.
Welcome to the forum! Nice revolver but I also see what looks to be a dent to the barrel other than that it looks great. If you plan on shooting it I would have a gunsmith check it out to make sure everything functions ok and its safe. I do that with any used revolver I obtain.