Here's some thread details that I compiled to go along with what others have posted.

FRENCH 34.7 mm X 1mm (1.366” x 25.4 TPI)

British 1.370” x 24 TPI (34.8 x 1.06 mm)

ISO 1.375" x 24 TPI (34.9 x 1.06 mm)

ITALIAN 35 mm X 24 TPI (1.378" x 1.06 mm)

French threads have the smallest outside diameter. ISO, British and Italian freewheels will start to thread on but will bind up after a few turns. If those freewheels are forced on they will damage the metric threads and can spin off under load destroying the hub.

ISO and British freewheels are interchangeable.

ISO freewheels fit Italian threaded hubs.

Italian threads have the largest outside diameter. Italian freewheels can be used on British threaded hubs but should not be switched back and forth with British freewheels.