British WWI military mail in Egypt: 30 AU 1916 stampless long OHMS stationary cv. ex APO ...

British WWI military mail in Egypt: 30 AU 1916 stampless long OHMS stationary cv. ex APO SZ 24 (Mustafa Pasha Camp ALEXANDRIA) to Paymaster CAIRO, tied by unrecorded 2-line SCDS - similar to Proud-D2 which was in use 1 day (14-09-16) & dated "OFFICERS BASE DEPOT" violet oval cachet (FB-unlisted); passed numbered censor - this rare on mail marked by military cachet; f/s same day BAPO Z (ALEX.) transit & b/s next day APO S 10 (CAIRO) arr. using Proud-D4 5 days before its LKD of use; slit open at top.