The primary chaincase outers are common to singles and twins so you will have the correct outer. Was there a single in the collection of bikes? If so, then maybe the person trying to put the single together is finding that the primary chaincase inner doesn't fit either, because it is the one which you should have. But if there wasn't a single in the collection you are down to the AOMCC, Ebay or Draganfly. Provided that the chaincase inner has the kink you can see in Adrian's photo and the hole for the footrest through it, you will find it is the correct part.
The oil seal is the same as for a Square Four. Dragnafly should be able to supply one.
36 4G, 37 VH, 54 KH(A), 75 Healey 1000/4, 52/53 ex ISDT KHA (project).
Former Machine Registrar & Archivist, General Secretary and Single Spares Organiser (over a 25 year period).
Now Archivist once more - but not Machine Registrar.