The mount may not have had 4 screws,it was common to use taper pins,in addition to a couple screws.This made a more solid mounting that would not move.It is possible the tapered holes were tapped for plug screws.There might be signs of this,inside the action.Taper pins are avaliable is different sizes,so one of the blank bases mentioned above can be installed with two screws and two taper pins(sized to clean up the old holes).Of course this is if taper pins were used, originally.
Mike