VIN is best, check this link
http://mx5cartalk.com/forum/viewtopic.php?f=18&t=34871if you are still unsure have one of the mx5 mechanics in your state take a look at it if it isnt already on their books
Australian Delivered *First Series of NA6 MX5s
*must have first section of the VIN that reads JMONA306100XXXXXX
Name/State/Built Month & Year - VIN *JMONA306100XXXXXX
Unknown/NSW - VIN ************011
mx52nv/WA/Sep '89 - VIN JMONA306100100162.
Unknown/NSW - VIN **********237
Australian Delivered *Second Series of NA6 MX5s
*must have first section of the VIN that reads JMONA306200XXXXXX
Name/State/Built Month & Year - VIN *JMONA306200XXXXXX
Imported Eunos Roadsters
Name/State/Built Month & Year - 6U9000NA6CEXXXXXX