(To JS/07 M 378
This Marble Monument
Is Erected by the State)
He was found by the Bureau of Statistics to be
One against whom there was no official complaint,
And all the reports of his conduct agree
The epigraph to the poem is a parody of symbolic  Tombs of the Unknown Soldier,  which are located all around the world. 
 It seems that “JS/07 M 378” is how the Unknown Citizen is identified: initials and numbers are cold and impersonal, but can be quite convenient for bureaucrats. (“JS” may hint that the citizen had the ultimate anonymous English name, John Smith.) Also, the title is the only mention of the  unknown citizen  and serves as an allegory or extended comparison to something outside of the poem. 
 
 The number represents a theme that runs throughout the poem: the loss of individuality. The mystery man was never mentioned by name, but only by a mere number. This portrays a certain insignificance. The poem describes his life as something so conventional that he didn’t separate himself from the crowd in any way thus having no specific personal identity. This is also a fact that pleases the government because this way he wouldn’t get in the way of anyone. He is just another man lost in the crowd whose life bears no meaning to the bigger picture. Perhaps the unknown citizen isn’t only one individual, but someone who represents unknown people in general about whom little can be said.
The Unknown Citizen