Here is what I know about the trigger stops. Most adjustable sighted K and L frames were machined for the trigger stops, and back in the 70's to around 90's they were installed from the factory on all the revolvers, not just the target hammer/target trigger models. There were times when these stops could loosen and they would make the gun not fire. Many smiths would simply remove the stops, which are held in by one screw, and most shooters would never know the difference. That is why a lot of the revolvers would have the cut out for the stop in the frame, but no stop present.
I am not sure what revision of the different models did away with the trigger stops.
I personally use them on all my target guns (adjustable sights), just my preference.
No non adjustable sighted K or L's that I have seen have the factory stop.