Those big hubs were straight off the road bikes, only really suitable for road use ( Impala ) , they did away with them after 12 months with smaller hubs given to the mk2 in 1969 and were used throughout the 247cc range. However in reality those big hubs were not fazed out until late 1970. Also now the Mk2 had alloy fork yolks and not steel as was previously used and lighter fork stanctions making the bike a lot lighter.
Below is one of my Mk1 models of 1968 prior to be rebuilt to original spec / finnish etc and a 1969 mk2.