I believe it may be somewhat based on who the engines came from. I have 6-71 TI's from Covington who, I believe, are out of business.
I was amazed to find that I only have one pencil zinc for each engine. It's on the outboard side cooler, bottom rear, 1/2" by 2" long. There is also a plug on the raw water pump for each but I was told several years ago to not use it as it eats up the impeller as it wastes away. Everything else is bonded to a shaft float and protected by the shaft zincs.
Hope this helps get you started and good luck.