Have you measured the dimensions of the receiver hitch under your Mercedes? I wonder if it is built to the international metric units. Also measure accurately the ball mount that you are using. Maybe it is the standard 2 square.
The ball mounts are certainly "slip fit" into the receiver, but it seems like the amount of clearance your photo shows is excessive. Certainly welding and grinding would close the gap.
WBCCI #8607 VAC Region 11
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