IRD is not Italian threaded. You'll probably have to go 2d hand if you want to stick to Italian. Or you could thread an english threaded freewheel on your hub. You'll have to be careful as this will cut the threads a bit. Sutherlands says this is a "B" fit; it works but you will not want to go back to Italian after this. I did this recently on an old campy hub I wanted to keep using. It's working great.
http://www.interlocracing.com/casset...eels-567-speed
Are you certain that your hub is Italian threaded?
Are you looking for 5, 6, or 7?