The 1960-D WDDO-001 is the strongest one:
You maybe looking too hard for something that is not there? When you find a variety, they should jump right out at you.
Your coin is a small date cent. The WRPM-001 is a large date. All of the small dates have that mark on the inside of the 6 loop.

Even the proof cent has it this year. The same engraving mark.
(I just realized it was not the 1960-D RPM-001, so I edited my post) 
Edited by coop
10/02/2015 12:14 pm