For the purpose of section 26 (6) the name of the author of a cinematographic film may appear in any sequence or in any frame of the film, whether the name is visible or not when the film is shown as a moving picture: Provided that such name shall be preceded or followed by the word 'copyright', or 'outeursreg', or 'kopiereg', or the symbol (c), or (o), or (k), or the words 'all rights reserved' or 'alle regte voorbehou', or any other obvious or ordinary symbol therefor or abbreviation thereof, either with or without letters or digits indicating a date.