Hello dr sportster
It is a solenoid switch. That is what I called these things for many moons. This one is internally grounded. Inside there are a few parts. A spring loaded bar that makes contact between the two lugs. A metal cup with hole in the center for the plunger to act on when the solenoid is energized. A coil winding with an iron plunger that moves against the spring loaded pin and bar assembly that make the bar contact the two lugs. Thus transfer the battery power to the other lug energizing the starter.
Ground for the coil is accomplished with a spring, form the ground side of the coil to the matel portion of the solenoid. Ones the unit is mounter, it will make the ground for the coil.
Ignition power to the coil will activate the solenoid.