Ok, just to resolve your confusion a bit, there have been 2 types of trigger stops employed on S&W revolvers.
The first type was a tab that mounted in the recess for the trigger and was positioned and held with a small screw. Unfortunately, the screw tended to shoot loose and the trigger stop either didn't function as a stop anymore or it got out of position and jammed the trigger. That type of trigger stop has now been completely eliminated from S&W revolvers but can be found on some of the older Target models.
The second type of trigger stop is what is now used on most of the N frames and some of the Performance Center L frames. This is a small pin that fits inside the rebound spring and blocks the motion of the rebound slide, thus functioning as a trigger stop because the trigger is directly linked to the rebound slide. Now, one thing that I believe is a side benefit to this new type of trigger stop is that it helps to keep the rebound spring from "buckling" and binding in the bore of the rebound slide. I say this because every single N frame I've handled has the least stagy trigger of any S&W I've ever pulled the trigger on. Currently, I have 3 of these stop rods and intend to put them into both of my K frames and my L frame 620. Which have all been tuned but are still just a tick stagy when compared to my N frame 610. IF those stop rods take that last remaining bit of staginess out of the K and L frame triggers it will confirm my hunch.
Now one warning about installing these stop rods is that you have to install them correctly. One end is flat and the other slightly pointed. Unfortunately, I cannot remember which way is correct but I do know that if you install it backwards it will cause the trigger to lock up depending on whether the barrel is pointed up or down. I "think" the pointed end goes towards the pin for the rebound slide but am not the least bit certain of that. Since I have a new 625JM coming I'll be doing an action job on it this weekend and this time I'll actually write down which way it goes when I pull the rebound slide. If nobody else posts the correct orientation check back Sunday and I'll have posted the correct orientation.