I stumbled upon this source for the difficult to find (in the US) 50mm trailer ball. Better yet the ball stud is 3/4", and not metric, so it is bound to fit your ball mount. No more asking your friendly airline pilot to buy some in Germany and having to evade customs on the way back. ;-)

http://www.etrailer.com/dept-pg-Ball...eter_Ball.aspx

Price: $8.95

Product Specs
50-mm Diameter Ball
1-1/2 Inch Shank Length
3/4 Inch Diameter Shank
3500 lbs GTW
Chrome