My boat does not have the one in Dag's picture, which is inline on the raw water
intake line upstream of the pump. But it looks handy to have such a fitting for winterizing. I might add one in my reassembly.
Here is an exploded diagram of the heat exchager setup that I modified to correct some errors and to indicate anodes:
There is a 5/8" plug
which has an anode and as mentioned above, needs to be sawed down to a length that fits. At the other end of the HE is a 1/4" plug which on my unit had the remnants of an anode. So I bought one and put it in there, too. That makes for two anodes in the HE.
Apparently there are no anodes on the block itself. I guess since only coolant/antifreeze runs through the block an anode is not needed.