You have a faulty Dual MMD cartridge which is no longer available, but you can replace the fitted stylus with a substitute.

Step 1: Unstick the adhesive with a sharp, thin blade inserted between the stylus and the housing.

Step 2: Flip the stylus down from the front and remove it. Insert the new stylus, first pushing the plastic nose to the back of the cartridge, as shown here:

Step 3: Then flip the stylus forwards into the cartridge using light pressure. Now you're done!