You are buying a rare vintage solid brass belt buckle/fob made in the U.S. by Basic Tool Supply in the 1970s featuring the Great Seal of the United States.  Please see details below:

"The Great Seal is a principal national symbol of the United States. The phrase is used both for the physical seal itself, which is kept by the United States Secretary of State, and more generally for the design impressed upon it."

"BTS was a manufacturer of sold brass and German silver buckles founded in 1974.  The company started out producing leather crafting tools which is the origin of the company's name.  They quickly transitioned to making buckles and operated under the acronym "BTS."

BTS also produced belt buckle "blanks" than were marketed to artists and used to create one of a kind buckles.  They were often filled in with exotic hardwoods, stone inlay, resins, and carvings.  They also collaborated with Steven L. Knight to produce a series of wildlife themed buckles.

The company was located in California and in later years (1980s to early 1990s) made buckles under the names Sunset Brass and Diablo Valley Associates.  They also produced buckles for government agencies and police departments under the name "The Buckle Works."

This very detailed scrimshawed centerpiece featuring the Great Seal of the U.S. by an artist certainly seems to be one of the belt buckle "blanks" that were marketed to artists in the 1970s to create these one of a kind buckles.  This buckle/fob could have also been created to commemorate the Bicentennial of the U.S. in 1976. 

The orientation of the Great Seal of the U.S. may indicate that it was made by the artist to be used as a fob.  The end of the buckle would be in a vertical position and a ribbon would be used to display the piece.