The pins that hold the sight on should be finished flush with the side of the rib on the barrel and blued afterward, so that they almost disappear. For that reason, I would say the front sight on your Model 19 has been replaced. It's entirely possible it might have been replaced by S&W, but I wouldn't think so. Likely it was replaced once the gun was out in the field. It is not a difficult operation as long as you are not trying to make it look like factory work. To do that, you would have to remove the barrel from the frame, install the new sight, fit and polish the pins flush, then re-blue, and re-install. It's a bit of a job to do it the way the factory did it.