I believe one mounting screw enables the regulator ground to be functional. The other screw just helps retain the regulator. If you remember the side the screw was missing from and examine the regulator you might find out if you had a good ground or not.
If the screw that establishes the regulator ground turns out to have been the one missing remount it with both screws and try it. Clean the regulator ground area as well as the area on the alternator housing where it contacts. Also since the screw was missing I think the regulator has been changed out before. Some of the aftermarket type regulators of this type were not particularily reliable. After about 3 good regulators or a lot of miles the slip ring inside can groove pretty bad from wear by the brush contact. If you can see the slip ring in there maybe have a look at it's condition.
They start out smooth right across when new.
A good rebuilder can change the slip ring out. Even I keep the slip rings in stock for some bosh alternators as it is not unusual to run into very high milage examples of them.
I just had a look at your picture and assume the ground side had the screw. Unfortunatly there is no quick test for a regulator of this style except substitution of it. Somewhere on site will give the amount of brush wear permissable.
A good sign on yours is both brushes are worn evenly. Beware when pulling a regulator that has one brush much shorter than the other. I cannot remember the reason for this at the moment. Only that it is indicative of an underlying condition.