Quoting Struc:
Yeah, it doesn't come up in CAPS because CAPS only shows the full front or rear arm as an available part. In fact, that part number doesn't come up with any models. Maybe the newer Mitsubishi parts program has it in there (can't remember the name of it at the moment).
In Mitsubishi ASA(New CAPS) it doesn't come up. I believe it doesn't come up, because if you order new arms they come with the ball joints already installed.
When I ordered new Rear lower arms, I thought I would need to get ball joints for them, but when they arrived, the ball joints were already in and ready to go.