(b)cause each such article to be marked with the prescribed mark, or with the prescribed words or figures denoting that the design is registered; and, if he fails to do so, the proprietor shall not be entitled to recover any penalty or damages in respect of any infringement of his copyright in the design unless he shows that he took all proper steps to ensure the marking of the article, or unless he shows that the infringement took place after the person guilty thereof knew or had received notice of the existence of the copyright in the design.