As
@deerstalkert said it is a unique adjustment of the Spanish locks for the trigger sear engagement of the tumbler.
It's use has been discussed here on our forum as priority to those locks since my membership.
It's use is one of three(3) trigger balance adjustments available with the Spanish made guns such as CVA and Traditions Arms and others.
Just as our current T/C, they have typical failings/issues of older guns that may need to be addressed.
Unique to the CVA/Traditions of the T/C is the shoulder on the hammer, that shoulder allows the lock can be actuated fully in hand without damage, it wont over travel.
So my advice to you, with your experience, is look closely at where that screw contacts the sear. Then adjust the screw in/out while working it in your hand, so that you can see how the adjustment was meant to be used.
One common failing, is an over adjustment that has stripped the screw threads in the tumbler by a previous owner,, and a loose fitment in the tumbler that lends the screw to travel, aka; it's need some loctite when the best position is found.
All of this has been discussed in detail for over a decade in many-many threads.
