[McMurdo]

agreed, their calibrators like yours and the mV and resistance versions are well known in calibration circles. They were the way to do things before integrated digital computerised calibrators came about and are often found in factory labs and workshops. That one there for example might have been used to simulate a 4-20ma signal transmitter in some industrial instrument such as a pressure sensor or modulating valve motor.
Don't drop it, they're over a grand new