From what I can see, the rear sight appears to be a standard pre-war target sight. When you look at it in person it should look like this:
They were so nicely fit and finished with the gun, that a replacement is something that should be easy to see. Also, if the rear sight is removed, it is also SN'd to the gun on the bottom (the little itty bitty screws make removing the sight challenging without buggering up the screw heads).
As to the front sight, that is an anomaly. Usually it would be a Paine or Thin Narrow blade, or a Patridge sight. Like you suggest, the round front sight is much more consistent with what you would see on a fixed sight gun. The photos are not clear, but the blade still appears to be pinned, so it could be replaced if you did not like the way it shot. The height of the blade seems to be consistent with the standard blades I see on the M&P Target guns.
You can check for a barrel replacement by seeing if the barrel serial number (on the barrel flat) is the same as the SN on the rear face of the cylinder and on the butt of the gun. With all that said, my guess, based on the use and worn nature of the gun is that it shipped as a target model with one of the standard front sights, the owner carried it in a holster and did not like the sharp front sight cutting up his holster when he drew it, so he replaced or modified the front sight to make it easier to draw... It also could have been a special order, as the factory would do that back then. A phone call to the dealer could solve the barrel replacement concern and a request for a few more close-up photos of the sights could allay some of your concerns, give you some more info and help you make a better decision.
Good luck!
Richard