That is what I am trying to figure out. I am assuming the engineers are calling shell plate that would mate the two materials. For instance, if the shell was 20' in height, the lower 5' would be 6Mo and the upper 15' would be 2205. I have never seen anything like this called out before, so I wanted to make sure there is no material out there that would be a mixed alloy of the two materials or something strange.