Probably hard chrome and it's aftermarket, I think one company called it Armaloy. Very durable though. Smith&Wesson never did a hard chrome finish either before or after sale. Grips are aftermarket too. I would be leary about wether Smith did the trigger job too. One lie leads to another.
model 13 threads always catch my eye. seeing this makes me thankful I already have a 13-3. I bought it ftf, paid more than I wanted but it is a nice piece and no regrets. chrome to me is meant for bumpers, even though we no longer live in that era. I see chrome inside barrels I cringe. I'm sorry to rain on this. I have my weaknesses.
Whoever did it, and whatever it is, it looks pretty good. And of course the 3'' 13's are mighty fine and desirable revolvers, and one of my personal favorites.