Thank you for your post. As a result of confirmation, although MX6G50B can be used as a gearhead of the M6RX6G4Y, use of the MX6G50B as an old model number: M6RA6G4Y gearhead will be outside the manufacturer's warranty. (The gearhead that is published by the manufacturer that conforms to M6RA6G4Y is M6GA M or M6GA B. Any number is entered for □ and for the combination of M 6 R X 6 G 4 Y and the gear head M X 6 G □ B depending on the gear ratio The direction of rotation will change.When the direction of rotation changes, it is necessary to change the wiring.) Regarding the combination previously used, please confirm again though excuse me.