Dan C. Johnson
West Jefferson, North Carolina
They are called carbine sockets. They came out during the Civil War and were used well into the 1880s. Almost any cavalry carbine fit into the socket. It had a belt loop and was adjustable.
If you have a question, write: Ask the Marshall, P.O. Box 8008, Cave Creek, AZ 85327 or e-mail him at marshall.trimble@scottsdalecc.edu