I'd venture a guess that the entire arm and part of the back is from the eye's previous owner, or at least that the arm uses that chakra. Just consider how tired Kakashi gets using his sharingan. What if he was able to use it with the previous owners chakra though? Perhaps it would be a better fit. Perhaps the arm is needed to form the seals using said chakra.

Also if you go back a few episodes you will see the arm and the weights are different from the Raikage's. Still don't know what their purpose is, but the chakra outline you see is a bit different from the look of the actual weights. I wonder how well the Hokage can form seals with what appears to be a rather crippled arm, or if it is just an act to hide the form of the arm.

Suishi... isn't that the name of the guy Itachi was accused of killing?