I only use OE (that means bought from MB)for the head gasket and the valve guide oil seals. I recently purchased a Elring, Goetze, and OE gaskets for 104 motors and closely examined them.
The OE gasket had thin shiney stainless inserts in the area next to the place where the gasket seals to the timing cover seal. The original gasket and the aftermarket replacement did not have these. These inserts looked to have little teeth cut into them and its my impression that they are designed to stake the gasket to its original installed position. I presume that some amount of recession is responsible for the common leaks.
I don't think the 103 motor is as critical but gaskets and seals are so small a part of total costs that I never take the chance.
Bosch Master, ASE Master, L1
33 years MB technician