Ah, the 67mm chamber which was a result of the allowable tolerance of the length of the chamber being 2.5mm(65mm, 70mm, 75mm) before reproof. I think it was the 1939 rules that made this stipulation which revolved around the empirical data that revealed that for ever 5mm increase in chamber length a corresponding increase of pressure of 1400 was found.

Kind Regards,

Raimey