Mailbox on a navy ship-ABC Wednesday "M"

Touring the USS Yorktown, a floating aircraft carrier museum at Patriot Point in Mount Pleasant, South Carolina is a lesson in history.  On one of the self-guided tours below deck I noticed this U.S  Mailbox.

April is Poetry Month: Excerpt from "Night Mail" by W H AUDEN

Asleep in working Glasgow, asleep in well-set Edinburgh,
Asleep in granite Aberdeen,
They continue their dreams,
But shall wake soon and hope for letters,
And none will hear the postman's knock
Without a quickening of the heart,
For who can bear to feel himself forgotten?