"I now know where you mean. T
"I now know where you mean. That hole is not used on the MD11C but it is the same cover as is used for the older single cylinder engine where it is the cooling water inlet to the manifold (on the MD11 that's half way along on the underside) I don't know why they used the same cover ~~~~ I spent ages trying to unblock a non existent hole in the manifold to line up with it!
I can assure you there is no mention of an anode in owners manual, workshop manual or parts list.
There is no anode and one fitted where you suggest would serve no useful purpose whatsoever."