Section 28H. In any trial under section twenty-nine on an indictment found or a complaint made for any offence committed after the filing of a proceeding under section twenty-eight C, the fact of such filing and the action of the court or jury thereon, if any, shall be admissible in evidence. If prior to the said offence a final decree had been entered against the book, the defendant, if the book be obscene, shall be conclusively presumed to have known said book to be obscene, or if said decree had been in favor of the book he shall be conclusively presumed not to have known said book to be obscene, or if no final decree had been entered but a proceeding had been filed prior to said offence, the defendant shall be conclusively presumed to have had knowledge of the contents of said book.