The closest SN I have to S117429 is S1164xx which shipped in 9/54. The N-frame cylinder is larger in diameter than the K-frame and requires a different speed loader. Just by the appearance of its finish, it seems to be what S&W called the ".357 Magnum" later becoming the Model 27. There was a very similar model called the "Highway Patrolman" (later becoming the Model 28) which was more utilitarian and sold for less.