In your photos is one picture of the schematic of the gearbox and the gearbox pump/control valve setup.
THe dwg. clearly shows the output port TO the heat exchanger and the return port FROM the heat exchanger.
THose are the two 1/2" ID lines seen in the photos of the gearbox pump and the control valve.
Those lines should go ONLY to the gearbox oil cooler likely mounted on the side of the engine block near the stringers. THey will have NOTHING to do with anything else. They are strictly for cooling the gearbox.
They will disappear below or near to the engine underside. The seawater will also run through that heat exchanger/oil cooler to cool the gearbox oil so also follow the seawater circuit as suggested. Your system is VERY similar to mine although a different engine and a similar gearbox, mine a MG506. TD followed very similar designs.
THe zinc you are looking for is for the gear oil cooler, not the gearbox itself.
There will be ONE zinc in that gear box oil cooler on the end cap. It's a small one and if like mine be very carefull about the length of the zinc when you replace it or you will damage the cooling tubes. If the cooler is anything like mine it is expensive to replace and is basically not available any longer. Cut it 9/16", no longer, from the shoulder where the zinc pencil screws into the cap. Check the zinc often, every 50 hr or so untill you determine the needed interval.
When you find the gear cooler post a photo and I will tell you if it is like mine.
The gearbox AND the gear cooler are under about 300 -400 PSI when operating which is why the hoses are actual hydraulic hoses with hydraulic fittings..
THe other hoses, the tank and the other pump driven off front of the engine by the belt should have nothing to do with the gear box. Those hoses also look larger so maybe supply several items. Steering, bow and or stern thruster, fin stabilizers, hydraulic anchor windlass, lifting boom or dinghy lift? ?? THis system maybe or should also have an oil cooler but NOT tied to the geabox cooler oil circuit at all.
The steering is maybe less likely to be operated from this system. It can be hydraulic but not powered.
I could be wrong of course but you need to carefully trace the hoses to be sure. Make a dwg. but be carefull tracing as it can be quite easy to mistake one line for another if they are in close proximity.
But I seriously doubt that the gear box and the other hydraulic system are tied together in any manner except that they each have an oil cooler for their own, individual needs, with the only connection between is the seawater supply line.
