A cartridge is formed of a mounting plate having a geometric configuration with an upper edge, a lower edge, a left edge, a right edge, an interior surface, and an exterior surface. A higher plate above the mounting plate has a geometric periphery with an upper edge, a lower edge, a left edge, a right edge, an interior surface, and an exterior surface. A plurality of posts couple the interior surface of the mounting plate and the interior surface of the higher plate to create a space for the receipt of at least one pickup. Two spring biased plates are mounted on the mounting plate. A coil spring separates the two spring biased plates. The spring biased plates have locking fingers reciprocally mounted on the mounting plate for engagement with locking projections on a docking ring. Each spring biased plate has a finger hole for retracting the locking fingers.