Sheldom has the skinny on threading... search Sheldon Brown headset threading at
www.harriscyclery.com ISO and italian 1" are close enough to be intechangable. The recent vintage Colnago straight blade steeel forks I have dealt with have been ISO. Your 105 HS has ISO threading. If your old campy HS should be Italian and you use it on an iso steerer,then you shouldn't go back to an italian threaded steerer with it.