Magnum M1 intake mod??

Hello from the future. This is the one and only post I've found that addresses the function of that 2nd water port in the M1 dualplane.
I'll be installing a MP 5.9 Magnum in my truck soon and have been wondering if people have utilized that water port. Is it meant to return coolant somewhere that's not pressurized? If its plugged the carb heat simply deadheads under the thermostat may take longer to warm up.