unauthorized signature placed on a security prior to or in the course of issue is ineffective except that the signature is effective in favor of a purchaser for value and without notice of the lack of authority if the signing has been done by
(a) An authenticating trustee, registrar, transfer agent or other person entrusted by the issuer with the signing of the security or of similar securities or their immediate preparation for signing; or
(b) An employee of the issuer or of any of the foregoing entrusted with responsible handling of the security.