I think the box is the auto shut off valve, that kills water flow to the membrane when the product water line is shut off, like when using a float switch. That way it doesn't continuously run waste water down the drain.
I removed the original restrictor and got an inline one to match the membrane size from bulkreef when I replaced my membrane.
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John
Current Tank Info: Reef: 65RR (6x HOT5 Nova Pro). Fresh water: 55 tropical
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