benz i havent had the privilege of talking to any mb engineers but i would bet a nickle against a donut hole that the eng located the port on the pump for convenience(so you could see it and be able to insert the light tester) and all that worked out to 15 deg atdc and its just a bench mark. no diesel engine could develop max power with the fuel getting to the cyl 39deg atdc (15+25)
just a experienced guess
larry perkins
71 old cars