Welcome to the THR,
It was a post WWII thing on the sights but remember that Smith & Wesson fabricated revolvers out of parts bins (which is why some parts have assembly numbers and letters often mistaken for serial numbers. Yours was made approximately from 1954-56 by its serial number sequence. So you have a late pre-model 10 apparently. Numbering models did not take place until about 1957 or so. Some specialists will be along later that I am sure will give you more.
You can also see oddball out of sequence older parts used on newer guns which can happen when you allow for special orders or the factory wanting to get rid of surplus parts stock. The no. 10 became the workhorse firearm issued for the policing industry which cared often about having a firearm at a decent price rather than the specific makeup of a firearm. S&W tried to give the customers what they wanted even if it might cause collectors to puzzle over the oddball variations later.
The long and the short of the posts below is that the older Smiths allowed the half moon front sight to be carefully bent to deal with windage like the old SAA's. New manufacturing techniques post war allowed precise machining of the revolver barrels with the front sight in perfect conformity with the rear and so the old half moon bending trick was no longer needed.
https://www.smithandwessonforums.co...esent/7512-model-10-front-sight-question.html
Regarding the different coloring, due to long service lives of these things, it is possible that the barrel is not original to the gun, that someone reblued it, that the holster/container affected blueing on the barrel, your revolver was made up at S&W from a mix of older and newer parts, or that you are seeing cold bluing, etc. If you get a whiff of sulfur, it is probably cold bluing along the barrel to dress it up but that test is not perfect as some cold blues do not have that odor.
Leather Holsters were hard on the barrel sides and the cylinder causing finish wear on the finish in those places. I notice that your barrel and cylinder appear closer as a match in color than the frame which might be an explanation. Leaving a revolver in a leather holster that has whatever leather treatment in it can also cause some color changes long term in the bluing.
As long as the revolver is sound mechanically, the K-Frame is a historic and well built firearm that will serve you well regardless of the parts issue. The one that you are showing has a nice finish and hopefully a good bore and cylinders. The good news is also that there are a fair amount of gunsmiths that can work on your K-Frame versus a Colt of a similar vintage and spare parts are readily available.
You have a classic revolver made during S&W's peak time for excellent manufacturing techniques, modern steels, and good finishing. Enjoy it.