I think headspace was ok before and after - problem is the chamber for 7x57 is a little bit bigger at the end than the 7x64 reamer will cut of
see the base measurments for both cartridges

7x57

7x64

is not so much but enough that the 7x64 will fireform in an ex 7x57 chamber. what a proof house was maybe tolerating still in the 70s would be today a reason to refuse the rifle. I don't think that there is a risk, its very little and the case will blow out peaceful. many shooter probably never notice this if you don't show it.

to make a 7x57R into a "7x65R" today you take the 7x67R Luyven which was created in 2003 to take this problem.

7x67R

you see it have a bigger base diameter and give a fine clean chamber in a 7x57R barrel