The PPM meter, with the black background and the white scale from 1 to 7 was a standard in UK Broadcast for decades. It was used to establish audio transmission levels and line up.
Each scale number from 1 to 7 represents 4dB, with PPM4 corresponding to 0dBU and PPM6 corresponding to 8dBU. The meter was incredibly easy to read: Speech should peak to PPM6, and maximum transmission should not exceed PPM7, where overload LEDs would often light, and the transmitters would become over modulated.
It uses a taut band movement on a moving coil milliameter, for fast response and slow fall time, which was tightly controlled by a standard BS6840. The EBU used the same scale, but with dB markings instead of 1 to 7.
The earliest drive circuitry was valve based ( yes it has been around that long ) , but solid-state cards were designed to generally fit on the rear of the meter. Early drive cards used precision metering and calibration circuitry with many presets. Later cards embedded the calibration into a small microprocessor. The attack, decay and calibration are closely controlled, so that any particular meter would react the same to music and speech the same as any other.