1918 WWI unfranked British army cv, UK via Egyptian PO; LKD postmark, odd route

British WWI military mail in Egypt: 22 IV 1918 stampless OAS civilian stationary cover from CAIRO to LONDON, tied by CAIRO-K postmark - late use in EEF period & with officer's signature; backstamped same day APO SZ 10 (Cairo) transit using large Proud-D7 postmark & marked by "Passed By Base Censor EEF" red handstamp (FB-PBC4); slit open at top. 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).