Could it be that the reason the Countess wore a countess coronet rather than a other grandchildren's coronet was cost? There already existed a countess coronet available, and maybe it was felt that the expense wasn't warranted for only one use? Although why the Earl would have purchased a new coronet and not one for his wife would seem a bit odd. Maybe he used an already-existing coronet? Does anyone know whether he had a new one made, or borrowed one?