Still sizing em 4.47 Rob, anything else don't touch the heads in most batches.
Been saying for years in fact most producers inc H&N are the same and don't measure whats on the tin. Not been using JSB that long but only ever come across three batches that measure whats stated on the tin, first was from 2002.
Lets just say manufacturers know when they've made a good batch!
The die may start out a certain size but in high demands I reckon the multiple dies going in to each tin (unconfirmed) are sped up. You should all know what temp does to dimensions.
Its not only the head diameter that's out the lengths can be different as well approaching 0.5mm.
By the way best groups ever repeatedly seen were with Maggies when they did measure 4.52.