Keyboard.Row6 is constantly held floating during lightpen access (isn't HIGH because it's open collector)
Keyboard.Colums aka PSG.Ext.PortA are configured as inputs aka HIGH-Z
The above LOW and HIGH (or HIGH-Z) levels seem to be misused as power supply GND and 5V. There seems to be no power-supply required for the CPC version (unlike the ZX Spectrum version, which connects to cassette EAR port, and which requires an external 9V battery).
* As seen on the package and advert, both the LP-1 and CAD-MASTER light pens work with '''colour monitor only''', ie. not with green screens.