Theoretically there were no '71 Challenger R/T convertibles offered or produced. And theoretically, an N-code 383-4bbl was the largest engine available in a '71 Challenger ragtop.
However, it is not improbable that one or more got produced - either via a special order for a Chrysler executive or otherwise slipping through the cracks. After all, everything required to build one would be on-hand at Hamtramck.
IIRC there was a JH23R1B VIN on file with the NHOA over 20 years ago, (yes, JH, not JS) though the status and/or location of said car is either not known or not widely known.
There is precedent for other theoretically non-available combinations that managed to get produced, including several known 1970 Barracuda 340 hardtops (BH23H0B). The 340 was to be exclusively 'Cuda (BS23) according to all documentation.