In the photo below, the upper image (1915 DWM) has an unrelieved sear bar, the lower (1917 Navy) has been relieved. The white arrow points out the position of the sear bar bevel on both guns.
As you can see, the bevel on the 1915 is right up against the safety bar, which prohibits the upper from moving backwards and so the action cannot be opened with the safety on.
The bevel on the Navy is relieved--cut back (actually, forward) far enough to allow the receiver to move to its full rearward position without intersecting the safety bar.
--Dwight