The difference is that back then the brass cases were electroplated. Current nickel cases are electroless nickel which seems to make the cases more brittle. I had 2 boxes of nickel RP brass which I got as factory loads in the late 1970's which I loaded until the yellow brass was bleeding thru the nickel and the primer pockets were so loose that you could almost reprime with your thumb. New cases are no longer like that.

Bruce