Porsche Carrera RSR Rear Fenders - Openings

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Last weekend, at a charity event, I purchased a taxi ride in a replica 1975 Porsche Carrera RSR-IROC (#5 Unser) at Sonoma Raceway (CA). I wanted to understand the car a bit better, so I did some reading in Ludvigsen's "Excellence Was Expected." It didn't answer the question as to why some RSR's have holes in their rear fenders and some do not. When did they first appear? Is this a factory variant? What is ventilated? Engine? Brakes?