Balboa Water Group 53974-05 PCB, Balboa, EL2001, M3, 53974-05If you are using this board to replace a Mach 2.1 Version board, you will have to move the circuit board stand-off's inside the control box. The following topside(s) are compatible with the above circuit board: 58-138-1050, 58-138-1052 Balboa circuit boards are marked with a “Chip Number” that is located on a white label placed on the logic chip near the center of the board. It is imperative that this number is supplied to Horizon Spa & Pool Parts for proper circuit board match. Just because boards may look similar the programming may not allow it to work properly in all applications. Please review our Balboa Chip Cross Reference below.