Final answer:
In 'Fahrenheit 451', Montag and Faber used the green bullet and a similar device by inserting them into their ears for secret communication.
Step-by-step explanation:
In Fahrenheit 451, Montag and the old man, who is later revealed to be named Faber, used the green bullet and a similar object to communicate secretly.
They inserted these devices into their ears.
This device, known as a 'green bullet' or 'seashell radio,' allowed them to speak to one another discreetly without being overheard by others, particularly by the oppressive authorities in their dystopian society.