Section 12A. If the holder of such mortgage fails to file in the assessor’s office a sworn statement of all his estate liable to taxation under the preceding section, including a statement of the full amount remaining unpaid upon such mortgage and of his interest therein, the amount stated in the mortgage shall be inclusive as to the extent of such interest; but his interest in such real estate shall not be assessed at a greater sum than the fair cash valuation of the land and the structures thereon or affixed thereto; and the amount of a mortgage interest in an estate divided after the creation of such mortgage need not be apportioned upon the several parts of such estate, except as provided in section seventy-eight A. Whenever, in any case of mortgaged real estate, such statement is not brought in, no tax on such real estate for the year then current shall be invalidated for the reason that a mortgagee‘s interest has not been assessed to him.