500 Two-Stroke Tech

Hi All, I have an import with a 51X chassis, my V5C however gives the chassis as 5AX - I understand some bikes used an A in the prefix possibly for some reason of Yamaha.
Does anybody know what the meaning of the A really is? Was it perhaps something peculiar to Japanese paperwork that has crept onto the V5C at the time of UK registration.
Many thanks, John