« Reply #10 on: Wednesday 12 July 17 19:55 BST (UK) »
It seems likely that Isabella Haig Stevenson and John Duncan weren't married - at least at the point of your father's birth. Without reintroducing your father's name to the thread, SP shows a 1938 birth in Banff for a xxx Duncan, and another index entry (with the same reference number) for a xxx Stevenson, which suggests he was registered as xxx Duncan or Stevenson, and his parents weren't married.
You'd need to see the birth certificate to confirm that - if the parents were married, the date and place of the marriage should be recorded.
Ruth
McArthur, Milne, Mitchell, Black, Robertson, Morrison, Slessor, Lawrence - Aberdeenshire/Banffshire. Muir, Waddell, Fraser, Orr, Cowden - Lanarkshire/Renfrewshire/Dunbartonshire. Dalziel, Dalzell, Gourley, Cromie, Crombie, Bell - Co Down. Lewis, Corrigan, Morris, Cox, Hay - Monmouthshire/Pembrokeshire. Baker, Ginger, Woodhurst, Swift, Jones - Kent/London.