Section 21. Requirements of notice of registration of trade-mark. - The registrant of a trade-mark, heretofore registered or registered under the provisions of this Act, shall give notice that his mark is registered by displaying with the same as used the words "Registered in the Philippines Patent Office" or "Reg. Phil. Pat. Off."; and in any suit for infringement under this Act by a registrant failing so to mark the goods bearing the registered trade-mark, no damages shall be recovered under the provisions of this Act unless the defendant has actual notice of the registration.