I'd guess supplied by GM and probably went back to them. It's pre Cobra.
https://www.motortrend.com/features/1962-corvair-endurance-test/The plate looks like Transport rather than Dealer or Manufacturer - I wonder if Petersen had their own plates for testing unregistered cars at that time? ...a transporter plate authorizing the limited operation of a motor vehicle ...... must have proof of financial responsibility. DMV was strict on who could use plates. Manufacturer and Dealer had to be used by owners and employees only.
Now the real question - did Gurney drive in the rain?
In November 1961, editors from several Petersen Publishing magazines, including Hot Rod, Motor Trend, Sports Car Graphic, Car Craft, and Rod & Custom, plus some pretty heavy hitters in the racing community, gathered at Riverside International Raceway for what Hot Rod described as a "24-hour high-speed economy run" using two bright-red '62 Corvairs. And we do mean heavy hitters: It speaks to the power of Petersen's empire that guys named Unser, Gurney, and Shelby were on hand to help shepherd these two little coupes around Riverside's winding 2.7-mile road course for what would be a "grueling 1,549.1-mile ordeal."
Carroll Shelby conducted the test, acting as "sports car racing expert," and in his capacity as the chief of the high-performance driving school, he hosted at the track.