(b)in the case of a building or land not assessed under clause (a), the gross annual rent for which such building (exclusive of furniture or machinery therein) or such land is actually let or, where the building or land is not let or in the opinion of the [Board] [Substituted by Act 24 of 1936, s.69, for "Cantonment Authority"] is let for a sum less than its fair letting value, might reasonably be expected to let from year to year:
