163. Where an offeror as defined in section 131 becomes an insider and through purchases effected through a stock exchange or in the over-the-counter market becomes the beneficial owner, directly or indirectly, of voting shares of a corporation carrying twenty per cent or more of the voting rights attached to all voting shares of the corporation for the time being outstanding, such offeror, within three days of acquiring such twenty per cent of the voting rights, shall file with the Commission a report as of the day on which he attained such percentage.