[broadgage]

The 110 volt DC supply might suggest a power station application, that supply being widely used for controls and instruments in power stations.

Perhaps some form of remote control for a large dynamo, by selecting one of many possible field currents.
Or perhaps a large fan drive with a variable speed to ensure optimum combustion of coal under varying load.

Unusual though, since most such remote controls used either a pair of push buttons, one for "up" and one for "down" or a three position rotary switch biased towards the center position for "hold steady" and turn anti clockwise for "reduce" and clockwise for "increase"
The engineer would hold the rotary switch against spring pressure, or hold down the push button whilst observing instruments, until the desired result was achieved.