How do you have the router oriented? If you’re facing the tool mount head-on, the on/off switch should be on the left, the power cord exiting the router body from the right, and the DeWalt label to the front.
I just mounted my 660 this AM, it has those same two ridges and I know the mount works.
If you notice the mount is keyed to fit the slots in the nose of the DW660 – i.e. it will only fit on the mount 2 ways. The mount would go on either the left or right side of the small slot you have visible in your picture.