Bob, does the fitting look like the one in the attached picture?
If yes, then it is part number 000-500-08-94 and it is a $3 part.
On my sold 1990 300E, there was a slow leak on that fitting. My mechanic very carefully removed the fitting and replaced an O-ring. He commented about being careful with the clips on the radiator to hold that part as if that breaks, then a new radiator is needed.
My feeling is that you got a different part as the one you had broke was on the radiator.