Hopefully the seller would be able to back up the "...supplied new in Japan in 1970 to a member of the U. K. armed forces in 1974 it came to the U. K. where it has been ever since." and "...a genuine RHD example."
But I would not dismiss the story out of hand. One of my own cars was ordered new from Nissan's diplomatic sales office in Japan by a USAF pilot who used the car in Japan and then took it back to the USA with him. He was then deployed to a UK base and brought the car here. He sold it here when he rotated back to the USA. Hey presto, Japanese market car in UK.
There were also RAF pilots who were deployed to Japanese USAF bases on exchange programs, so an RAF pilot stationed in Japan is also possible.
It would be unusual for a purchaser to order an early HS30 from Nissan diplomatic sales office in Japan, but not impossible. Most such sales were either for Japanese market variants to be used in Japan, or North American market variants to be used temporarily in Japan and then brought back to the USA gratis as chattels.