Although primarily a venue for sports car racing, Riverside International Raceway also hosted a Formula One US Grand Prix (1960), NASCAR events, motorcycle races, and the "Rex Mays 200" -- for Indy Cars!
 

Even though these shots are far outside the scope of this website, they provide a spec- tacular view of high-powered open wheel racers in Riverside's "esses".  Martin identifies the first four cars as "Gurney, Clark, B. Unser, Foyt".   What year would this be?

The next lap:  Somebody overshoots Turn 4.  Is Dan Gurney still leading?  I have no idea who these drivers are as I never attended a "Rex Mays 200".