This meter patent filed around 1979 pre-dates Parker by quite a few years, and you can see from its write-up that the basic operational ideas have been borrowed: US4216424. Carl Vette put this into production with his Creative Electronics, and changed things a bit from his original application. The schematic, schematic write-up, and advertising lit with a picture of the meter can be found here:The circuit uses an interesting "peak-to-peak" detector among other things and looks more like a real design. Apparently Vette did a lot of consulting with local universities, contributing his expertise towards precision electronics fixturing for mostly biological research experiments- not bad for a repairman.
A few years later Terry Cook patented US4795966 for Sencore, and this one looks a lot like the Parker meter.