I don't believe ALL flex discs have that German word. Only some must face a certain way, and I don't believe 124's are (at least for a diesel). When I took mine off to remove the driveshaft, I found three bolts that did not even have washers and I did not have any vibration. If a bolt is loose, that is very prone to vibration. Since air guns don't get torque evenly distributed, I would check on the bolts first. If any are loose, they must be fixed.
MB dealer part for flex disc includes the bolts, washers, and nuts (again, at least for '87 diesel). If aftermarket was used, it may not have included the new bolts.
But, I would check that the two flanges are seated properly to the flex disc and that bolts are toqued as appropriate.
Its also possible that the center support bearing mount was loosened, moved, or the 41 or 46 mm bolt that tighens the driveshaft halves together was loosened and not put back correctly.