Thread diameter of hub
Presumably the Shimano hub/freewheel has an ISO freewheel thread and the Campy hub has a Italian thread. These thread standards are close enough to interchangeable.

From Sheldon Brown's Freewheel Page:


  Italian: 1.378" x 24 tpi / 35 x 1.058 mm
  
  ISO: 1.375" x 24 tpi / 34.92 x 1.058 mm