The Car Designation used above refers to the letters printed on the cars by Bachmann to aid coupling the cars in the unit in the correct formation.
Non reversible units 1938, 1956, 1959 and 1962 tube stock have a rubbing plate in the A car and a central sprung buffer on the D car. Units can be mechanically coupled A to A, but the electrical connections on the coupler needed o insulating material between tem. To couple D to D, a D to D adapter had to be used as the buffers prevented the Wedgelocks even touching. All D end Driving Motor Cars carried an adapter under one of he transverse seats. This would have been particularly important on the Northern Line, where at times over half the fleet could be "the wrong way round.
This is also true of O, P and R stocks.
Later stocks could be coupled mechanically either way but not necessarily electrically
S stock have purely mechanical Wedgelock coupler at each end.