I also haven't taken the rear sight off to see if the assembly number is the same, but having owned a large variety of target model Smiths, I am pretty sure that it isn't some revised version, it has the factory target front sight as well. The rear sight is a perfect fit across the top of the frame.
The gun may well be an original target. A factory letter is so easy to request,
costs next to nothing compared to the value of the gun, and answers all the
questions about the gun. It will do away with all the needless speculation.
On the other hand, Roy has told us, over and over, that , for example, the
factory could refinish a gun, and NO ONE could tell. That is how good they were.
Additionally, there are numerous examples of target revolvers that ,for all the
world, look correct. The rear sight leaf will, indeed, be a perfect fit, and the
guns will have a factory front target sight. But - they won't letter that way.
There are also known guns that were originally shipped as M&P's, returned at some
later time unsold to the factory, converted to target, and sold a second time.
They will letter as targets ; sometimes a letter will reveal this full history.
Later, Mike Priwer