In Fahrenheit 451, what is written on the firemen's helmets?
In the future society of the novel, a "fireman" is not a person who rushes to the scene of a fire to put it out. Instead, they are the censorship arm of the government, and they travel to places where people are suspected of keeping books to root them out and burn them with flamethrowers.
With his symbolic helmet numbered 451 on his stolid head, and his eyes all orange flame with the thought of what came next, he flicked the igniter and the house jumped up in a gorging fire that burned the evening sky red and yellow and black.
(Bradbury, Fahrenheit 451, Google Books)
The number 451 is not Montag's identification number, but the temperature at which paper ignites; all the firemen wear these helmets, not for personal protection but to identify them as firemen, in charge of destroying books. Montag is literally tagged by the government, his purpose in life chosen for him, and everyone can see who and what he is at a glance.
"With his symbolic helmet numbered 451 on his stolid head, and his eyes all orange flame with the thought of what came next, he flicked the igniter and the house jumped up in a gorging fire that burned the evening sky red and yellow and black". Fahreinheit 451 represents the temperature at which books are burnt. The fire has two connotations in the book: it represents the destructive force on one hand, and the purifying blaze on the other hand, in order to evidence the antithesis between good and evil, life and death, civilization and barbarism. The fatidical temperature is written on everything : firestations, firemen's helmets, tanks , and it becomes the major symbol of Bradbury's novel.