Why did they add load type to the headstamp?
The load was added for the simple reason to identify it. The earlier 1941-43 production was Mark Iz with a velocity of about 1200 fps. The velocity of the Mark I was then increased to 1250 fps and although the headstamp was unchanged, packages were marked “V 1250”.
In 1944 the Mark IIz was introduced with a velocity of 1300 fps (although it had been approved in September 1943), but early production was still on cases originally intended for Mark Iz and so had the same headstamp. There is no way of definitely identifying late Mark Iz from early Mark IIz once it is out of its packaging, although all 1944 production should be treated as Mark IIz.
From mid 1944 the cases were headstamped “IIz” and from 1945 “2z”.