The Pentek 154006 is the correct replacement cap for the following units:
The 3/8" Black Cap for Slim Line and #5 Blue Housings is an in/out black cap for Pentek Slim Line housings. This Pentek filter housing features a 3/8" inlet NPT/outlet connection and is not manufactured with a pressure release button. This 1/4" housing is also manufactured with durable reinforced polypropylene.
3/8" Black Cap for Slim Line and #5 Blue Housings specifications: