My understanding was/is that in 1915 the tolerances were changed due (mainly) to experience in Palestine.
Supposedly the original SMLE bolt diameter minimum (-) was changed to become the standard, the standard dia was changed to be the max (+) diameter acceptable tolerance.
At the same time the standard bolt race diameter was changed to be the minimum (-), and the max (+) was changed to be the standard diameter.
This is supposed to be the reason why pre-1916 SMLE actions were preferred as the donors for the Australian HT program during WW2 (because they are toleranced tighter, not because they "Flexed better").