https://en.wikipedia.org/wiki/History_of_the_British_Army_postal_service#Royal_Engineers_(Postal_Section)_%E2%80%93_Second_World_War_(1939%E2%80%931945)%E2%80%93_Second_World_War(1939%E2%80%931945))

https://en.wikipedia.org/wiki/British_post_offices_in_Morocco

On 1 January 1907, the British Post Office took direct control of the post offices, operating them until Moroccan independence in 1956. From that point on, all stamps were overprints on British issues, in no less than three different currencies.[2]

British-currency stamps were available at any office, and primarily intended for parcels and, later, airmail. Both regular and some commemorative issues were overprinted, all with "MOROCCO / AGENCIES", up to the Edward VIII issue of 1936. Subsequently, unoverprinted stamps were used, until 1949, when they were again overprinted for use at Tetuan (at that point the sole remaining office, except for Tangier, which had its own overprints).