One vintage Weston 506 type L volts SC meter 0-5V. My dad modifies to expand the scale to add 10 volts to any reading. Has mounting bracket that would work under a dashboard. It now reads 10-15 volts. The red wire is positive and the mounting bracket is negative as would be used in a car. I hooked Hp up a 9 volt battery and it read on the 1 or 10 volts. Probably for use in a car.