Presumption as to documents: prosecution-tendered documents are presumed genuine unless rebutted and may be admitted unstamped. When a document produced, seized or received in proceedings under the Act is tendered by the prosecution against an accused or jointly tried person, the court shall, unless the contrary is proved, presume the truth of its contents and that signatures or handwriting are those of the persons they purport to be; documents otherwise admissible may be admitted notwithstanding that they are not duly stamped.