My best "guess" is that the M102 engine block was also used for carburated versions and was based on older Mercedes blocks, so the adjustable mount on the distributor was inherited from the old design. As you noted, changing/rotating the distributor will alter timming for a few seconds before the fuel injections' ECU alters the timing to correct it back to "normal."
The other "holdover" from past applications of the M102 block is the cutout in the front of the cylinder head that was used for the cam gear driven hydraulic pump for the SLS suspension in the wagons (Estates).
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