If you have two sewer ports to connect a sewer hose to, then one valve is for the galley and usually it has its own separate sewer port. The second sewer port is for the black and shower/bath sink drain and those other two handles allow those tanks to drain out the 2nd sewer port. When pulling the 2nd grey tank handle, it doesn't drain the grey tank into the black, it simply drains it out the sewer port.
A simply way to tell which handle is which... put some blue food coloring in the toilet and run some water then pull the handles one at a time and see which one allows blue liquid out.
Put some red food coloring in the kitchen sink drain and run some water down the drain and see which handle lets out red liquid... and so forth.
BTW... those handles ARE somewhat marked the chrome handle has a B near it so I'd assume that is the black. The first handle has a G and I don't know if that stands for grey or galley?
The farthest back handle I don't see any lettering but there is a decal there. I can't read what it says.
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