Thanks Sven, good to have more info on the subject.
The reason Rob asked was because I'm sure you know that if you actually measure the head sizes from a selection of pellets from a tin, you'll get quite a big variation, and rarely get an average figure of the size quoted on the tin, effectively making the quoted head size a meaningless number.
I know I have a tin of 4.52's and a tin of 4.51's in my pellet crate at the moment where the 4.52's are much smaller than the 4.51's. The 4.51's are a tight fit in my barrel and the 4.52's literally fall out if I hold the rifle pointing upwards.
The only thing I take for granted now with JSB's is that tins with exactly the same numbers (die/batch/headsize, everything!) then those pellets all seem to perform the same.
I can't rely on one die 34 being as good as another die 34 from a different batch even if outwardly they look exactly the same.
Perhaps they should change the pellet name to "JSB not as Exact as you'd hope"