Darryl,
I am NO expert in Japanese I can just about manage English, but I think it maybe the test certificate for use on the road
in the UK ours is called an MOT test which has to be done every year I think in Japan it is a lot stricter but it is not yearly
this is a guess but as it is dated with the mileage at the time makes me think thats what it is
I am sure somebody who speaks Japanese or has seen one of these before will be able to confirm or put us right
Frank