I believe that the sideplate number is an assembly number that will match the numbers on the frame and the yoke. That number is different to what is supposed to be on the butt. Technically, it is not legal to own a modern post-1898 revolver that has had the serial number removed. There would have been a number on the butt when it left the factory. What number is on the rear of the cylinder? I believe that you might find 13,531 on the rear of the cylinder as well as on the barrel, so that is your serial number, not the one on the sideplate. So 13,531 could have been on the butt at one time making it a 3rd Model as already noted.