An interesting box rarely seen today. Internally, it’s just an attenuator and a tunable bridged T network. Operation is simple: Select 400 Hz or 1 KC. Apply signal to the input terminals at corresponding frequency. View residual on a ‘scope. Fine tune oscillator/source frequency for minimum deflection. Adjust the balance control for minimum deflection. Repeat if needed. Percent distortion is peak-to-peak output voltage divided by input voltage times 100. Unlike HP’s active distortion analyzers this does not suffer from ground loops. The 320A can also be use as an attenuator. I am not sure when this was first manufactured but it is mentioned in the August 1951 issue of The Hewlett Packard Journal.