May be authentic, not sure. This plate has the arrow hooks and is a left handed plate meaning the belt attachments are on the right side of the plate. Not near as common as the right handed variety. Just found out online that the prongs should be.
062 in thickness for authentic, so I added another picture showing the thickness. Good solid example of a different variation US plate. The item "Civil War Brass US Belt Buckle Possible Antique" is in sale since Thursday, April 14, 2016.
This item is in the category "Collectibles\Militaria\Civil War (1861-65)\Original Period Items\Belts & Belt Buckles". The seller is "inlandvalleycustoms" and is located in Winchester, California. This item can be shipped to North, South, or Latin America, all countries in Europe, all countries in continental Asia, Australia, New Zealand.