Ordering Description Product number Description: A6DS Tandem Master, high-speed (74:1) Product number: 0818970881 Description: A6DS Tandem Master, standard (156:1) Product number: 0818970880 Description: A6S Tandem Master, high-speed (74:1) Product number: 0818971881 Description: A6S Tandem Master, standard (156:1) Product number: 0818971880