The Nicholson Over and Under is a .44 caliber pistol, measures about 7.5 inches long, and made in London by the Nicholson family armourers. The over and under barrel design allowed for it to be fired twice before being reloaded. For source and more details, click here.
Images from: http://amhistory.si.edu/militaryhistory/collection/object.asp?id=533&printable=1