Section 20: Transmission by post of anything indecent, etc., prohibited.

No person shall send by post—

(a) any indecent or obscene printing, painting, photograph, lithograph, engraving, book or card, or any other indecent or obscene article, or

(b) any postal article having thereon, or on the cover thereof, any words, marks or designs of an indecent, obscene, seditious, scurrilous, threatening or grossly offensive character.