I have the same heat exchanger on my Perkins 4.108 and recently pulled it and the oil cooler to have them serviced. As Terry said, the plug you are holding is not the zinc. It is the high-point air bleed for your cooling system (It is not technically the highest point in the system but it is the quickest way to get most of the air out of after service)
There should be two more identical-sized plugs on the bottom of the unit, which are for winterization of the cooling system (i.e. for draining it). The zinc-containing plug, as terry mentions, is toward the port side, facing aft and downward at an angle. It is larger than the plug you are holding (see link below for photos) and it has a recess where new zincs can be screwed in to it.
Would highly recommend pulling the heat exchanger and opening up the end cap to visually confirm that the internal copper tubes are all clear. My zinc plug had long lost its zinc, the plug itself was galvanically-corroded in-place, the internal copper tubes were about 30% obstructed with scale, and there were plenty of little bits of impeller floating around in there to boot!
Here is a link to an identical unit - you can see all of the bleed/drain/zinc caps in the second photo:
https://www.mrcool.us/na001406-perkins-heat-exchanger.html
The Perkins cooling system is dead simple so don’t be afraid to take some ‘before’ photos and start pulling hoses and components...it’s all pretty straightforward. Coring the heat exchanger and oil cooler should run about ~$100 apiece at a reputable radiator shop.
These heat exchangers are beasts so you’re probably fine but if you end up needing replacements, I would not recommend ordering from Mr. Cool, the site linked above. These parts can be found for cheaper through Perkins suppliers.
Good luck! I have photos of the whole teardown and refurb of the units so let me know if you need more info and I’ll dig em out.