Jim,
Yep. Agree. They were camming the gages in on those Enfields and that fit is made possible by stretching steel. It was interesting to go back and re-read the parts of Hatcher leading up to the headspace gaging portion; that slamming a bolt will forshorten the case enough to change its headspace. This reminded me that when you look at the SAMMI specs for the .308, that the maximum headspace for the .308 case is 1.634", while the minimum headspace for the chamber is 1.630". The assumption that the action will have no problem shortening the case that extra 0.004" by forcing it to fill out into the chamber is inherent in that spec, and of course there is about 0.011" of extra diameter everywhere in the chamber except the neck. That's what gets filled.
If someone wants to download the relevent pages from Hatcher's Notebook on how to gauge headspace, I have a temporary download link to a .PDF file
here.
Nick