Since he mentions a bright red 0, that indicates a calculator with a LED display. The early calculators, which had either LED or vacuum fluorescent displays, c

Since he mentions a bright red 0, that indicates a calculator with a LED display. The early calculators, which had either LED or vacuum fluorescent displays, could use either a few R6 cylindrical batteries or an external power supply, like a laptop today. When close to a wall plug, it was normal to use it, because the lifetime of the batteries was modest, mostly due to the high power consumption of the display. While this was a disadvantage, bright and fast VFD displays were much more appealing than the dull and slow LCD displays that have replaced them a few years later. I believe that HP has used only LED displays at that time and not VFDs, because they manufactured the LED themselves, unlike other calculator vendors.