[Gary refrigerator]

Inside the refrigerator cabinet where the water comes in on the back wall:

There is a 3 inch long x about 2 inch wide plastic piece the water filter screws into on the front. On the back, there are two plastic water lines that go into the filter. One line is waterproof and fits snug. The other line doesn't "click" into the fitting. A plumber said it's missing a fitting. The other line doesn't appear to have one yet it's watertight. If I put the water line in, it sprays a mist of water pretty freely. There are no parts in the diagram that seem to be missing. Do I need to get a replacement plastic piece the water filter fits onto? We bought this frig after many shelves had been salvaged but I don't see any parts missing on this part of the water supply. Thanks!