The front sight - its was either modified from a halfmoon (most likely), or special ordered that way originally (not likely, but possible) or the barrel was replaced by S&W, and the correct serial number was stamped on the replacement barrel (again, possible, but not likely, and there should be a rework mark on the frame if so).
These revolvers often had the front sights modified, but without examining the revolver, or seeing detailed photos I am shooting in the dark (pun intended).
If I have time I will post a photo of a 5" M&P, late '30's production, that has had its front sight modified - I am at work right now and I am teaching this weekend so I may not be able to get to it until Monday. I believe it was a fairly common practice to modify these sights, at least its not all that rare. A lot of guys in the inter-war period shot various types of bullseye matches and also often modified the grips, the rules for these matches may have had a "service" type category that limited entry to fixed sight revolvers only, especially in police competitions. Cost may have been a factor also, modifying a standard M&P locally was probably significantly cheaper than ordering the target variation.
A factory letter will specify when it left the factory, where it was shipped to, and the finish, barrel lenght, types of stocks (grips) and any special features it had - for $30 you can't beat it. As to its value, I wouldn't count on it as a retirement asset, but it is a neat, interesting old S&W, and well worth having. Now if you should happen to come across J. Edgar Hoover's S&W RM, serial number 1, you may be able to retire tommorrow.
Edit to add - consider CW's offer to help with the photos, I just can't make out the details enough on the ones you posted.