Will a letter from S&W document whether or not a firearm has been sent in for refinishing or repair?

No.
A factory letter staes:
Date shipped
Where to- dealer name or individual or gov. agency like a P.D.
Caliber and/or Model Number
Barrel Length
Finish
Type of grips
Usually the extras like TT, TH, TG, Sight Options.

__________________
Regards,
Lee Jarrett

S&W pistols usually have a star behind the serial number if returned for repair or refinish.

This is mostly only true with pre-war guns. If you have the name of the original owner/s you may be able to track something down that way as repair/refinish records are kept by customer name and not serial number

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.

I have actually requested a letter on a gun sent in for refinish. The refinish was not documented in the letter -- very disappointing to say the least. One of the reasons I asked for the letter was to document that it WAS a factory refinish.
Really, The factory letter is pretty much a form letter with bits of information added at the end Not worth the money spent in a vast majority of the cases is ny opinion.

Unless you truly think you are in possession of a historical or otherwise very particular gun, than the letter that you receive from S&W is pretty much a donation, as it doesn't really tell you too much. As far as the date of manufacture is concerned, that can be found in The Standard Catalog of S&W - Supica & Nahas, or by researching on line.