Its an OF1413, in Mike Davis SBS Book Pt 2:
In 1978, Singapore Airport Bus Services Ltd, was
established to provide an international standard public transport
link between Paya Lebar International Airport and the major hotels
along Orchard Road, the service commencing in July of that year.
the chosen vehicles were 15 Mercedes Benz OF1413/61 fully
air-conditioned coaches fitted with bodies assembled by Chua Heng
to the general outline of the West German-built Mercedes Benz O.303
luxury coaches which were popular in Europe at that time and having
the distinctive body styling of the marge, with smooth side panels
and large side windows whose upper quarter curved into the body
profile to meet the roof line. However, with their somewhat noisey
front mounted engines intruding into the saloon, they shared little
of the smooth ambience of the O.303s they resembled.
