A thread about setting the IP in 615/616 engines says to line up the groove on the IP 2 teeth before top dead center and to have the crank timing set at 45 degrees BTDC. I tried that on my car and it worked. I believe my engine is from ‘83 - so won’t take the locking pin - and every time I tried to install the IP at 24 degrees and right on the notch (rather than 2 teeth off) it would always skip. I think 45 degrees and two teeth before the notch takes that skip into account and according to that thread, it was the method specified in the FSM for those engines. That thread is here: W115/OM616 Injection Pump Installation and Timing How To.