Whilst David is undoubtedly correct in the guidance he has given, I doubt that it is possible to fit the front section of the fully enclosed rear chaincase to an earlier (1954-55) chaincase inner. The shapes are very different but it was possible to buy a bike without the FERC, which is why David's advice will work without one. Inner chaincases from the VH, VB and NH are identical for 1956 - the end.
36 4G, 37 VH, 53 ex ISDT KHA (project), 54 KH(A), Healey 1000/4 (project)
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.