More Bill Greene photos; most of these are shot in the mid-60's at Oceano Dunes (formerly Pismo Beach), which are on the coast, and are still accessible to motorsports. 
This photo shows young Bill holding onto family friend Phil Eaton; the photo was probably taken by his mother, riding on the back of Bill's father's bike. Another Triumph, a 500cc model, TR5 probably, in 1964.
And this is a Matchless GC3 'Trials' model, ridden by Dick Eaton (brother of the Phil above).
That's Bill in his mom's lap, again in '64. I love the 'mouth organ' badge on these early 60's Triumphs; both are unit-construction 500's, set up for dirt riding.
Bill on the back of his father's Triumph; riding in deep sand like this takes considerable skill!The bottom photo shows Ronn Blumke cresting a dune on his TR6 650cc Triumph. The dunes are blown continually by an eastward wind, and develop a sharp slope on the leeside (east slope); sand is often cascading down these steep slopes, which are called the 'slipface'. They're the best side to ride up if you want to catch some air at the top!
(All photos copyright Bill Greene 2008)