Hi, I have a document here that states the official diamensions for Mills Grenade rings and pins. The pin is 1.5 inches long and 0.09 inches dia. I am not sure if this means both halves of the pin or just one. The ring should be 1.5 inches outside and 1.22 inches inside dia. and made of tinned steel, this document is dated 1919.
It does not state if the ring is split or solid, I have both types in my collection, as well as brass pins too, which I am sure are original. If you think about how many different factories made Mills Grenades, there was bound to be differences in materials and quality etc, especially in wartime, hope this helps, Tony.