Fig.2 shows that the top (^) of the septum (S) is rounded after extraction.  It is difficult to make an entry with a pilot drill (Fig.3 red arrow); the drill easily slides into either the mesial or distal socket (short arrow).

Septum Slot

Xin Wei, DDS, PhD, MS 1st edition 01/02/2015, last revision 01/02/2015