earlier vehicles were only available in a limited number of standard colours, e.g. dark red, light red, dark green, to try to match but simplify existing operators' liveries. This just predated the decision by the National Bus Company to standardise on two colours: poppy red and leaf green. The timing was such that several vehicles were delivered in dark red paint to Ribble, who repainted them poppy red before entry into service. At London Transport's insistence, London bus red was added to the colour card, and their fleet eventually amounted to 506 examples, 69 of these being National 2s purchased for Red Arrow services, between 1973 and 1980. In 1978, Leyland brought out a simplified model, the Series B, intended to replace the Bristol LH which had been in production since 1967 and was a type popular in NBC companies for rural routes. It was available in a single length (10.3 metres; 33 ft 9 in) and had a revised interior that had minimal lighting and without the rear roof-mounted heating unit in previous models. Heating was under the seats and was basic but effective. These vehicles were lighter, and this characteristic and the lower cost helped make extra sales. London Country bought a large number of Series Bs, which other operators quickly purchased second-hand when London Country was broken up in 1986. The National 2 was introduced in 1979. It differed from its predecessor mainly by having a wider choice of engines, along with a new nose moulding giving it a more bulbous look and a grille to cool the new front-mounted radiator, and a revised rear end with new lights and a different engine door. Engines The Mark I Leyland Nationals had an 8.3-litre straight-six turbocharged Leyland 510 headless diesel engine. The Leyland 510 engine had an unusual design, in that it featured non-detachable cylinder heads; any work on the valves required the crankshaft and pistons to be removed to enable access from the cylinder bore. This engine did not prove popular with all operators, being prone to poor fuel consumption and heavy smoke production if not maintained to high standards. Some operators experimented with a different engine and found they could avoid the 510, which had the reputation of being a high maintenance unit and hard to work on. Later on a simplified model (10351B/1R) was offered, with an engine that was reduced in power to 150