My experience is than an Italian HS will go on tight on an English steerer, while an Engligh HS will be just a hair loose on an Italian steerer. So I doubt the English HS will have damaged the Italian steerer. And if it is an alloy Super Record HS, then any thread damages should be to the HS, not the steel steerer.
So if you want to install an Italian HS for correctness, I don't see any problem with that.
Big Spring, Texas, USA
> From: Sean Flores <email@example.com>
\r?\n> Subject: [CR] BSC/Italian Headset Question
\r?\n> To: firstname.lastname@example.org
\r?\n> Date: Sunday, March 22, 2009, 2:46 AM
\r?\n> i got around to finally disassembling my Colnago to check
\r?\n> the condition of the HS and BB. At some point, someone,
\r?\n> possibly the original owner, threaded an english headset
\r?\n> onto the Italian steerer. I know that the threading for
\r?\n> English/Ital is pretty much the same except for the pitch,
\r?\n> but I've been told it's not a good idea to go back
\r?\n> and forth from one to the other.
\r?\n> So should I a: replace the HS with another english one, or
\r?\n> b: replace it with the correct italian threaded HS?
\r?\n> Advice would be much appreciated.
\r?\n> Thanks all.
\r?\n> Sean Flores
\r?\n> San Francisco, CA