If the returning officer receives, before the close of the poll, an envelope other than a covering envelope and finds therein any document which it would have been appropriate to send to him in a covering envelope, he shall—

(a) securely close the envelope;
(b) endorse thereon, and sign, a statement that the envelope has been closed by him with the contents intact;
(c) place the closed envelope in a postal voters ballot box;
(d) thereafter treat the envelope as a covering envelope;

and any reference in this Part to a covering envelope shall be construed as including a reference to an envelope which has been dealt with by the returning officer in accordance with this section.