Fyi, some of the newer S&Ws don't have the lock.
S&W springs are easy to change and can make quite a bit lighter DA trigger. There are lots of videos on YouTube that cover about every model.
I've used the wolff spring kits on my S&W and on a Charter Arms Bulldog. Really easy to install and they make the trigger as good as my N-frame 357 that got it's trigger through round count.