OK, please forgive if this question sounds stupid, but are you sure you have a crush washer under the bolt?
Most newer W123's I've seen use two o-rings of different sizes, one small and one big, with no crush washer.
The older ones, like mine, use one small o-ring and a crush washer.
The part numbers I can find follow:
Crush washer: 007603018101
Big upper o-ring (usually found on newer models): 0009970048
Smaller lower o-ring (found on all models, same part # as o-ring for oil filter lid shaft): 0039975848