Dymwood Coupling

[dogsbody]

 My trailer (Circa 1971) has a dymwood coupling on it with the markings  30 cwt 2 Ball. I have been using a 50 mm ball and it seems to be a bit loose.
Do I need a 2" ball or Larger. If a 2 inch I can get a  U,S 2" ball . Will that work. I would really like to know the correct fit?

[brenten]

A 2" tow ball is 50.8 mm but when you buy one measure it first even though they stamped 2" some are in sized because of year's of use