Nonetheless, Mercedes-Benz engaged in intensive research and development of alternative propulsion systems in the 1960s. In 1969 the brand presented the OE302 electric bus. Two years later, a natural-gas-fuelled version of the O305 was unveiled, and in 1975 the OE302 duo bus went into trial operation in Esslingen, Germany. A minibus operated on pure hydrogen was demonstrated in 1977. In 1978 the hybrid electric OE305 was presented. The same year Mercedes-Benz do Brasil presented the OF-1315 front engine natural-gas-fuelled bus chassis for the Brazilian market. Other five duo buses went into trial operation in Esslingen the following year, two of them in battery/trolley operation and the other three in diesel/trolley operation. Also in 1979, Daimler-Benz sent a German-built O305 trolleybus to be tested on the São Paulo trolleybus system, Brazil, then one of the most extensive trolleybus systems in the western world. The O305 GT trolleybus would go into production in 1981. Also in 1981 a methanol-fuelled O305 was unveiled.