The SCCA USRRC years. The United States Road Racing Championship, which started in 1963, did away with all of the engine displacement categories that the older Sports Car Championship had and replaced them with a simple Over 2.0 liters / Under 2.0 liters / GT category. This category format, with regards to displacement, was also later used for the SCCA TransAm series; starting in 1966.
Did you know.............that Don Webster ran this Porsche 904 at Riverside in 1964 as part of the SCCA United States Road Racing Championship?

And what about the Huffaker Genie Mk 10B in 1965:

Or the Cooper Monaco:

Or the Elva Mk VII:

And, of course, the Porsche 718 RS 60:

Walt Hansgen's Lola T70 Mk1 in 1965:

Jim Hall's Chaparral 2A, also in 1965:

Ken Miles' Shelby Cobra:

And how about the Platypus Porsches......?

And the Merlyn Mk6:

How many remember the Bobsy SR2 and SR3 with Alfa or Porsche power; 250 bhp in a 1000 lb car...........?

Maybe we should think about an O-2 / U-2 class break for this proxy, too. Thoughts?
Allan