The engine rotates in the CW direction, so the 15 degree ATDC mark will be to the left of the TDC position--after top dead center (ATDC).

It appears as if the OM 617 IP is a completely different animal than the OM 603, so I'm afraid you need someone who has experience with those critters to help you out any further.

Once you finally determine where the ridge is, it might be helpful for future reference if you were to clean the surface and paint a stripe of bright coloring at that point.

Good luck...

BTW: I think you mentioned in another thread that you had the SIR Tools version of the lockpin. I've heard elsewhere that that the OEM MB tool works better than the aftermarket units.

YMMV