As mentioned here by several, the correct information is the car line code in factory parts manuals for a Road Runners is R. Car line code for Plymouth C-bodies is P. As the Factory Parts manual, clearly states, all ’71 383HPs (335hp), the standard motor for Road Runner, left the factory with a list number variant of the Holley 4160 series carb. Backed up here by direct significant dealer experience that dates to the time.