Where tax is chargeable under section 151B of this Act on an occasion after a reduction and the rate or rates at which it is charged fall to be determined by reference to the death of a person which occurred before that reduction (or before that and one or more other reductions) that section applies as if the Table in Schedule 1 as substituted by that reduction (or by the most recent of those reductions) had been in force at the time of that person's death.