Decades ago, when a customer found that my Tera Ohm Meter expensive, He suggested the specs for this low cost scaled-down model. This was found useful for people sub-contracting Telecom Equipment Manufacturing.
Even if we have Developed a High Precision System we cannot sell it in a Market that needs an Affordable Instrument with lower Specs. Three Types of the same Product for Three levels of Affordability and Field of Application. Get the idea?
This probably went upto 500V and 100V with a Toggle Switch and measured upto 1 Giga Ohm or 1000 Mega Ohms. The test voltage would be applied on the Switch, Connector or Magnetics to measure the leakage current in its insulation.
The leakage current would be converted to voltage, clamped with diodes and fed to the Vref of a 7107 which displayed the Insulation Resistance by doing a 1/x. Normally we use Vin to measure the voltage, in this instrument the Vin holds the Reference.