That trigger does not look like the original. On the original factory trigger, the set screws were smaller, and the rearmost screw was located closer to the second screw and acted as an overtravel stop by bumping into the back edge of the trigger housing that wraps around the trigger. The inner screw was a safety stop and was adjusted until the safety stopped the trigger from being pulled enough to release a shot, but it was still allowed to slide back and forth. That spring is not an original part, and where my old trigger (IIRC; I replaced it 20 years ago with a proper compound match trigger from Sharp Shooter Supply that is, unfortunately, no longer made as it didn't fit the Accutrigger housing) had a third screw further in that adjusted tension on the trigger return spring.
It looks to me like that spring is for adjusting the trigger pull within the limits of the spring. However, A WORD OF CAUTION on that design. Despite the additions, the trigger mechanism is still a basic sear override trigger. It doesn't have any internal levers to lighten a stout sear engagement, as the more expensive aftermarket triggers do. When you lighten that simple trigger toward the target trigger engagement level, the gun can fire as you close the bolt. I speak from experience with this. You cannot expect to get a truly light trigger. Indeed, that spring at the back may be chosen specifically to limit you to a trigger pull weight that won't do that.
All that said, it appears to me that there is room to put a very small set screw between those two adjusting screws so you at least have an overtravel stop, which can help trigger feel quite a bit. If the trigger is hard enough, though, you might need a carbide drill and tap to do that. In that instance, my approach would be to remove the rear screw, anneal it at about 1100°F if you have an oven, or just soften it with a torch (red heat without quenching) and then drill through the bottom of the hex socket for the biggest screw that slips into the socket freely or one that just barely kisses the flats of the hex recess, then heat and quench the threaded screw to get it hard again. A long, small-diameter inner set screw can then be turned into the threaded through-hole in the bigger screw's socket. I would do that after getting the big screw back in place for the spring and Loctite it at the right height. The small screw would be chosen to reach all the way down through the spring to touch the receiver to provide the adjustable overtravel stop.
