The N1254A-107 is an adapter from a triax plug to a triax socket. The shield is connected to the inner conductor on the connector side, and the shield is connected to the outer shield on the socket side.
This trick reduces the capacitive load on the signal-carrying conductor, since the inner shield has the same potential as the signal line.
This adapter is used to convert the "low" output of the N1294A-002 banana triax adapter from a Kelvin connection to a non-Kelvin connection.