I just measured 4 "1 inch" stems on 4 different bikes i have and found them in the 21.8 - 22.6mm range. I've never paid this any attention before, and never noticed a problem. This is not a Campy issue. The headset only relates to the OD of the steerer, whereas the stem relates to the ID. I'm not a vintage expert, but have never heard of this coming up before. Does your LBS have a pile of old stems they can try to see if there is some tolerance issue with your steerer?
Looking at some colnago pics, I don't see anything unusual in stems being used. just get a cinelli of your choosing.