The Liberty Bell is a cultural icon of the United States of America.
Background
The Liberty Bell was cast in 1752. Prior to the outbreak, the Liberty Bell had a place of honor at the Liberty Bell Center in Philadelphia, Pennsylvania.
Post Apocalypse
After Philadelphia became overrun with zombies, government forces tried to save cultural icons of American history.
The Westward-bound survivor group found the Liberty Bell on a flat-bed truck surrounded by zombies. They killed the zombies and took the truck. After encountering more zombies, the Bell broke free of it's restraints and careened down a street, smashing zombies as it tumbled.
Doc was really impressed by the result, wishing to see that again.
Gallery
The Liberty Bell on the back of a flat-bed truck