Swan neck tow ball 50mm

Any idea where I can get hold of a 50mm swan neck for my D2 7 seat. For some reason NZ has two sizes, 1 7/8" and 50mm. Just after one i can keep in the wagon and swap when needed. There must be a reason for the two sizes, but it makes no sense to me. All our work utes are 50mm, most private motors 1 7/8". Plenty of bolt through 50mm balls available, but too short to clear bumper and step.

I suppose I could a carry spare 1 7/8 coupling around, but that seems daft.