Oil Tank Dip Sticks

Hi Guys, just trying to understand the theory behind the dip stick upper oil tank level line "High" being 9.5mm lower on 74/75 model year. Was there an issue with oil blowing out at high temperatures

No oil blowing issues I'm aware of. But don't go to the high mark or over , midway between low and high seems best. If you fill to over , the excess ends up in the air filter or pollution control system. You don't want that mess or smoke.
But my understanding is there were 3 differing dipsticks and oil level marks stamped over the years with slightly differing lengths.. perhaps others know more.