Laserjet IIID power-up messages

If the "Menu" key is pressed while powering up my Laserjet IIID, the LCD
displays "SYSTEM? (+/CONT)". If the "+" key is then depressed, the LCD
shows "(C) LJIII/L 29".

When a Windows Printing System cartridge is inserted, the LCD alternates
between "(C) LJ III/L 29" and "Argilla Cart." when the "+" key is pressed.

None of these messages are described in the user or service manuals. Any
ideas?