I suppose that there might be good reasons for not registering a birth in 1917 during the confusion and social upheaval of war.
My understanding of the rules is that a birth can be registered after 12 months with the permission of the Registrar General, if evidence of the exact date and place of birth can be proved by documents or witnesses.
Having been granted permission to register by the RG, late registrations can be made in any registry office in England and Wales.
I don't know if records of the RG's permission for late registration are kept, but it might be worth contacting the General Register Office (GRO) to ask.
