Id say the barrel is factory correct imo. Rollmark size for the 686-5 was the last year of the small mark IIRC. I have a 2.5" and have been beating the bushes for over 2 years looking for one. Not an easy animal to find. Later variants, up to today, use a much larger size marking.

As to the sights, I suspect those were aftermarket installed, or ordered that way from Smith (PC was running an option then IIRC with either the 686+ variant perhaps?). So...perhaps the original owner upgraded the sights at time of order, or did an upgrade on his/her own later.

Not sure that helps you with regards to the sights...but I'd bet money the barrel is original to the model at time of manufacture. In any event, if you'd like a "true verified original" 2.5" 686+ I'll talk to you about a swap perhaps.