Road racing motorcycles accompanied SCCA races in California in the early 1960s.  500cc. British "One lungers" like the Norton Manx and the Matchless G50 were the dominant machines, driven by experts like John McLaughlin, Buddy
Parriott, Don Vesco, Tony Murphy, and others. 
  

Peter Adams on #90 Honda 350 leads John McLaughlin on #25 Norton Manx at Cotati on July 16, 1961.  McLaughlin was a frequent winner in this era.

McLaughlin and Adams  on the grid at Vaca Valley Raceway, on Aug. 20, 1961. I can't find a program listing for #190 so I have no idea who he is or what he's riding. Pete Kellond restored the ex-John Surtees Norton Manx.  See his beautiful bike HERE.