In April 1940, Imperial Airways and British Airways merged and were taken over by the British Overseas Airways Corporation (BOAC). In the same year, Italy declared war on England and France on the 10th of June effectively cutting off the Mediterranean from the usual BOAC route to Europe.
This interesting wartime cover from Labuan was auctioned in eBay in July 2016 and sold for £454.44. It bears the Malaya Straits Settlements stamps with a total franking of $1.65 and tied by Labuan postmarks 20 JU 1940. Endorsed “By B.O.A.C. as far as service p[ermits]” and bears the boxed censor cachet PASSED BY / 4 / CENSOR in violet ink. There are no markings at the back.
The rate of $1.65 was probably arbitrarily set (thus the endorsement) and does not match with the standard postal rate for available airmail services to UK within this timeframe. It falls short by 75¢ for the BOAC to Hong Kong for connection with the Pan-American Airways to UK (Air throughout) or even air-sea option. It overpays by $1.05 for the newly opened horseshoe BOAC route to the UK.
Therefore the route by which this cover was flown through remains somewhat a mystery especially with the lack of transit markings. The possible route could be:
The BOAC Horseshoe Route
Following the official declaration of war by Italy, the BOAC company joined the part of the African route from Durban to Cairo with the part of the Australian route from Cairo to Sydney. This new route from Durban to Sydney across 15 countries became known as the Horseshoe route.
The rate for Great Britain and Europe was now 60¢ for ½ oz. letter and 35¢ for postcard. The last through air mail for Europe left Singapore on the 2nd of June 1940; After this letters bearing the 60¢ rate were sent by air to South Africa through the horseshoe route and onwards by sea to Europe.
Short Empire S33 / S23 flying boats were used to carry mails in the new route.
The BOAC headquarter remained in Singapore until the Japanese began bombing the city frequently. On 30th January 1942, the Short S23 Empire flying boat Corio, flying from Darwin to Sourabaya was attacked by Japanese fighters near Koepang on Timor island. The flying boat, under command of Captain A.A. Koch crashed into the sea about 5 miles from the shore.
The situation worsened rapidly and by 14th February 1942, the last flying boat left Singapore and on the 6th February the shuttle ceased.