Baum Tools offers 41, 42 and 46 mm open end drive shaft wrenches but the salesman said there was no reference to the W124 application in his literature... per Baum, 41 mm is for 107, 108, 115, 123, 126, 201; 42 mm is for 201 drive shaft clamping.

Assenmacher offers 41 and 46 mm wrenches but again the sales staff was uncertain as to which fit a 124.

I measured 46 mm, which matches Engatwork's reply, so 46 it is!
Thanks.