It would almost certainly be Italian or ISO threaded and you could use either type of freewheel. It would be best to match it exactly if you plan to use the hub a long time as the aluminum hub threads are no match for the steel freewheel threads. You could put the 'wrong' type of freewheel on once but the hub threads won't be the same afterwards.

Did Campy make freewheels in 1990? If so, I wouldn't be surprised if they made their hubs and freewheels with Italian threads so that the Campy freewheels went on smoother than brand 'S'.