I could be wrong but I think it is a genuine error by the seller. I'm not saying the bike is good, safe or right but regards the 1971 registration, that could be an error or true if someone got an age related certificate, as the date of manufacture could be 1971 as per Hondas records. Remember the CB750 diecast that Frank had, that was imported from the states and the age related certificate came back from Honda stating 1968 !!!.
Anyway, I have wished the owner of the 350/400 good luck with the sale.