I wonder if the W123 is slightly different in that area? On the W126, I barely managed to get the 3/8" drive, 12 point socket to engage the fastener. It was really marginal and was not completely square with the head of the fastener. I got it done but would never advise anyone else to use it.

If the W123 accepts the larger 3/8" drive with no difficulty, the available space is definitely greater.