First time I have seen this as an entire envelope. Rogelio de Jesus showed me a fragment with incomplete left inscriptions  which at that time I could not figure out on what it was.

With provision for dead letter particulars at upper left but without any entries on this particular envelope. With the required entries perhaps the envelope was used for other official correspondence.

I think it is safe to assume that the envelope was used to inform the sender on undelivered or unclaimed (for registered letters) mail and instructions on how to claim a returned letter.