Old analog wattmeter 2 terminals
My best guess would be that you have only a meter movement that was meant to be fed by a power sensor.  The 60 mV probably refers to the sensitivity of the meter movement (it also appears on the bottom of the meter scale). Note that the full scale reading is 100,000 watts which, even at a 1000 volts, is 100 amperes. The terminals on the back of the meter were not meant to handle anywhere near that current level.