USA, FLATBOAT
CARRIED MAIL TO HENDERSON COVER 08-08-1985
A flatboat is a rectangular flat-bottomed boat with square ends used to transport freight and passengers on inland waterways. The flatboat could be any size, but essentially it is large, sturdy tub with a hull that displaces water and so floats in the water. This differentiates the flatboat from the raft, which floats on the water. - Wikipedia
This is a Flatboat