I recently purchased a 4" 24-3. The finish seems a little brighter than others that I have seen. Also, the trigger is a smooth target vs. a serrated trigger. I've seen the smooth triggers before, so I don't necessarily think that it's a replacement. Finally, the last four digits of the serial number are stamped on the rear of the cylinder under the ejector. I read a post by DOC44 that the serial number stamping could mean a return trip to S&W for some type of work. The gun is mint and it doesn't seem logical that someone would have refinished it or replaced the trigger, but stranger things happen.
Since S&W doesn't document returns in their "letters", the originality of this gun is somewhat undetermined. I have the original box, paperwork and tools. I'm not that concerned about the originality, just curious.