1918 WWI unfranked British army cv, UK via Egyptian PO w/Union Club cachet

British WWI military mail in Egypt: 28 AP 1918 stampless OAS "Union Club" stationary cover from PORT SAID to LONDON tied by PORT SAID-D postmark; endorsed by officer for free mail, cover marked by light green round Union Club cachet; backstamped same day APO SZ 22 (General Headquarters Lines of Communication, PORT SAID) transit. Unusual local routing from civilian to military post as most of the period military mail for abroad received by the civilian post office was not marked by a British military post office subsequent to overseas dispatch (although those marked like this generally originated from Cairo).