Wow! I've never seen a rifle barrel marked Smith & Wesson! I own a dozen Rolling Block Sporting Rifles, but none as late as yours. I think my highest serial number is in the 7,000 range. So none have the hump back receiver yours exhibits either.
As a S&W fan too, I'd be in heaven finding an old Rolling Block with that barrel fitted to it! Hope you end up buying it!
Topics related to Pre - 1898 Remington Rifles