56 models came out with a 12V negative ground system. It sounds like you've got an old 6/12 battery, one that used a Ford-style starter solenoid in a series/parallel arrangement to start on 12, and run and charge on 6 volts. The last of the 6V systems was the 55 1st. series, at least if it's a Chevy. 56 Fords had a 12V positive ground system, the one and only year for that one. In 57, the whole automotive industry standardized electrical systems at 12V negative ground. Freightliner trucks and a few other commercial vehicles kept using positive ground systems through the mid-1960's.
Jerry
