Yeah, ISO and Italian are close but no cigar. It's been a long time since I was a paid wrench, but I don't recall that they were ever easily swappable. It was more like sometimes. It would have generally been considered bad form to use the wrong headset, and it would not have been done on a good bike unless there were pretty compelling reasons.

While the late Sheldon Brown deserves his sainthood for compiling all the information that he did, AFAIK he was never actually a professional mechanic, so it is worth noting that some of his advice should be taken with at least a very small grain of salt.