All infringing copies of any work in which copyright subsists, and all plates used or intended to be used for the production of such infringing copies, shall be deemed to be the property of the owner of the copyright, who accordingly may take proceedings for the recovery of possession thereof or in respect of the conversion thereof:

Provided that the owner of the copyright shall not be entitled to any remedy in respect of the conversion of any infringing copies, if the opponent proves--

(a) that he was not aware and had no reasonable ground to believe that copyright subsisted in the work of which such copies are alleged to be infringing copies; or

(b) that he had reasonable grounds for believing that such copies or plates do not involve infringement of the copyright in any work.