Is this "sunken scew head" on the underside of a pan head a thing?
PhilippNagel's comment is correct.
The bottom of the screw head is flat except where it tapers to the threaded shaft.
This taper seems to be no wider then the outer dimension of the threads so a countersink would not be necessary. (unless of coarse you wanted the top of the head of the screw to be flush with whatever it is installed in but that would not really be the correct fastener for that application.)