Found this pic on a FB page called Ridiculous Radical Rides & Crazy Customs, there are several other pics where it was used as a "semi trailer bus" travelling from Damascus to Baghdad. The travelling accomodation included a kitchen and toilet. The original link is in Russian:- Для пустынных условий
"In 1932, for the construction of an oil pipeline from Kirkuk to Haifa, the Iraq Petroleum Co., which was engaged in the construction of the oil pipeline, purchased 4 American Marmon-Herrington TH-320-6 tractors, tractors equipped with 150 hp gasoline engines showed excellent results. themselves in the desert, which drew the attention of the Nairn brothers, whose company, Nairn Transport Co., was engaged in passenger transportation on the Damascus-Baghdad route - on the same roads where TN-320-6 tractors were used. In the same year, 1932, the brothers turned to Marmon-Herrington, where they ordered two tractors.
By order of British carriers, Marmon-Herrington manufactured two tractors: THD-315-4 and THD-315-6. Both tractors were equipped with a diesel engine with a capacity of 188 hp, equipped with an oversized cab, which had an additional compartment in which three people could be accommodated, equipment for working with a semitrailer, but differed in the wheel formula: THD-315-4 was two-axle, 315-6 - triaxial, respectively.
The passenger semitrailers were ordered from the American firm Bender Body. The semitrailer had 31 passenger seats, special compartments for carrying luggage, a small kitchen compartment and a toilet. Semi-trailers arrived in 1933, and then they were put on the Damascus-Baghdad route.
The buses made flights between Damascus and Baghdad in 18 hours, while developing speeds of up to 90 km / h. Due to the high travel speed, the route was popular with passengers, and due to its comfort, the buses received the unofficial nickname Desert Pullman. In 1936-1939. the modernization of the company's vehicle fleet was carried out - new 38-seater semi-trailers with improved thermal insulation were purchased, the three-axle tractor was also "modernized" - the second rear axle was dismantled. In this form, the buses were operated until 1943.
In 1943, the buses were bought by the British Royal Air Force. Air Force specialists increased the capacity of buses to 44 people. The aircraft were used by British aviators until 1945, after which they were returned (sold) to their former owners and returned to their usual routes. The buses operated until 1956, when, due to political changes in the Middle East, the Nairn brothers were forced to curtail their activities. Buses with tractors, like the rest of the company's property, remained on the territory of Iraq and Syria, and its further traces are lost"