The Dual Battery Switch 3 terminal is for metal frame ground systems, and the 4 terminal has an insulated ground terminal for use with fiberglass or other non-conductive materials.
These are Cadmium plated, non-breakable and with heavy duty housing. They resists rust and give a long life in most any environment.
Includes terminal nuts and lock-washers. Nothing else to look for or buy, saves time and money. Assures battery power for starting engine and provides secondary current for auxiliary power. Full time charging of both batteries. Gives you early re-use of secondary battery.