Rich Thorwaldson was one of the dominant riders in SoCal desert and MX in the late 1960s/early 1970s. After many successes riding for Steen's, the US importer of Rickman chassis kits, he went on to ride for US Suzuki for several years.
This ad shows him ahead of Jim Wilson, who was sponsored by Nick Nicholson Motors, the US Greeves importer, on 250 and 380 Griffons. Its amusing to see that Steen's ad person couldn't spell "Weslake" correctly.
cheers,
Michael