There is no difference as Ed said an MSD chip is nothing more than a resistor like what you can buy at Radio Shack. Before the digital boxes I made rotary dial two step boxes, the two step was nothing more than a relay to switch between resistors for a rev limiter and a low side for the brake. I built a couple different ones, one with a rotary switch with progressive resistance values to increase or decrease RPM as needed and the other with potentiometers which are variable resistors. I also got the resistance so I could set the low side below 3000 RPM which was the lowest chip MSD had at the time.