What difference does it make? Made in May 1970, shipped, slow boat, to Cal, and registered as a 71. Unless of course the little old lady is pulling a fast one with some re-plating going on.
Mine is a 1970, but built Oct 69 and it makes no odds either way on a 50 year old car.
If it were being sold as “ one of the last “, I’d understand. But surely, condition/price far far outweigh whether it’s actually what you US folk call a ‘71MY or ‘70MY as the differences should be minimal. Unless the differences between the plates make you think it’ll be worth a load less/be a hassle should you sell it on. By which time, you’d have the birth certificate and can point to the Ca DMV for any errors ?
Could the emissions plate be based on it being a (May) 70MY (changing emissions regs?) despite it being first registered in 71 as someone drove it off the forecourt?