I'm confused by this envelope!
Pre-stamped and 'By Air Mail', and pre-addressed to Base Air Post Depot, Karachi, so how would it have been distributed for use by Air Mail within India but outwith Karachi?
Jim
My interpretation would be if the sender didn't have/wasn't sure of a fixed location to send the correspondence to, BAPO Karachi tracked, as well as possible, postings and unit locations in India/SEAC. It was a clearing house. I believe similar arrangements existed for other theatres - eg in Cairo, for the Middle East.
Neil W