Garth-
If what you have measures 27.0mm for Japanese, English and Italian forks, you need a better caliper.
Some older English frames will be 27.0 or beyond, things were not very standard way back, but if it has had a Campagnolo Nuovo Record headset fitted, then it is smaller than 27.0. The Campagnolo tool case has a tool to check the gauge of the crown race and frame, get access to one or buy a good caliper that reads effectively in the tenths of a millimeter or finer.
When a writer discusses being qualified to fit a headset, they are probably also thinking about stack height, which can vary overall and the lower stack in particular that varies and is a wee bit more significant between extremes.
John Jorgensen
Torrance Ca