The provisions of this chapter shall apply to all taxable transfers, except as specified below, if the death of the transferor occurred on or after January 15, 1959, provided (a) if the transferor died on or after January 15, 1959, having made, prior to said date, an irrevocable transfer taxable under the provisions of the general statutes relating to succession and transfer taxes in force at the date of such transfer, then such provisions shall be continued in force, and the value of such transfer, calculated as of the date of the transferor’s death, shall be included in his gross taxable estate; (b) if the transferor died prior to said January 15, 1959, having made any transfers whether by will, by intestacy or otherwise, and whether revocable or irrevocable, taxable under former provisions of the general statutes relating to succession and transfer taxes, such provisions shall be continued in force for that purpose.