In Biggles in the Terai Biggles discovered that gold was being smuggled into India by means of an unidentified aircraft which flew the contraband and dumped it at a jungle airstrip to be picked up later by Holman Larta in his de Havilland Dragon Rapide. Neither Biggles nor Algy could recognise the gold-smuggling aircraft although, as Johns noted, there were few aircraft Biggles did not know.
The aircraft was described as "twin-engined, low wing, cabin monoplane" which had an unfamiliar cut about it. It was a military type, with piston engines, and had green, brown and black camouflage. It was armed with a machine gun which could fire amidships on ground targets, as Biggles and Bertie discovered at the jungle airstrip. The gun could also fire towards the rear, as it did at Biggles when he was pursuing it in his Hawker Hunter.
Among possible candidates, we can rule out aircraft from manufacturers from Britain, Western Europe or the United States as Biggles was certain to know about them. This leaves a number of more obscure possibilities like the ones listed below.
The Yak-6 was developed in 1942 as a twin-engine utility aircraft/light bomber for the Soviet Air Force. It had two 140 hp piston engines, carried a crew of two and had a cabin which could accommodate up to four passengers. Bombs or rockets could be mounted on racks under the wings. It also had a mount for a machine gun in a dorsal position which would have allowed it to fire on ground targets and towards the rear.
The Yak-6 is thus somewhat reminiscent of the British Avro Anson. It was used for ground attack and for communications/training purposes in the Soviet Air Force during World War 2 but was not, however, produced in large numbers, only about 300 being made. Not being one of the high-performance or famous fighter types, it is possible that Biggles may not have been familiar with it. The Yak-6 was supposedly retired from service in 1950 but the smugglers, whichever country they came from, could have got hold of an ex-service machine.
The DINFIA IA35 Huanquero was an Argentinian two-engine multi-purpose aircraft designed by a German team led by Professor Kurt Tank (designer of the Focke-Wulf 190). The Huanquero first flew in 1953 and entered service with the Argentine Air Force in 1957, serving in a number of roles such as light transport, air ambulance, photo-reconnaissance and bombing and gunnery trainer. Only 50 were built. It apparently could be armed with two machine guns although the location of the guns is unknown. Again, it is plausible that Biggles would not know about this obscure type, and the aircraft certainly had an unfamiliar cut to it, with the highset twin tail.