It does sound like an "R."
In the muscle car era of the 60's/early 70's, many (probably more than you think) cars were for racing. They came from the dealership, w/ No A/C, no radio, no console, etc, etc, etc. But had high performance 454's, 440's, 429's, etc, w/ 4 speeds, and 3.90 to 4.11...