Alright, I will spell it out.
The pointer mounted in the timing cover is a refernce point for lining up the point TDC; that is top dead center. In this case top dead center of the number one piston.
TDC is occuring when the "0" zero position of the balancer passes by. The direction of rotation indicates the by function the points before and after TDC. Expressed BTDC and ATDC. The degree wheel on most Mercedes, even ones where it can't be seen, has a scale mostly on the before side. You will notice if you face the motor and turn it clockwise the scale which starts at least at 40 degrees has marks every 5 degrees. As you keep turning and the "0" aproaches you are still before top dead center BTDC.
With a light you are illuminating only when the spark occurs allowing the marks to be framed at the moment. You need to remember which way its turning as it will look stopped. If the zero lines up you are at TDC, anywhere else you will be lined up with the scale and the number represents the degrees off TDC the the spark is firing before or after by position in time.
Bosch Master, ASE Master, L1
33 years MB technician