Establishment treated as a single unit for bonus computation; separate accounts can render a part a separate establishment. Where an establishment consists of different departments, undertakings or branches, all such parts shall be treated as parts of the same establishment for computation of bonus; provided that if a part prepares and maintains a separate balance sheet and profit and loss account for an accounting year, that part shall be treated as a separate establishment for bonus computation for that year, unless it was immediately before that year treated as part of the establishment.