Alternatively, the RSA website classify it as the 'date of 1st registration', so that would indicate it's first registration date in the UK for instance.....
https://www.rsa.ie/road-safety/road-users/vintage
How difficult can it be to have some joined up thinking and be clear on the rules FFS....
Presuming the E36, that's some annual motor tax saving in fairness (appreciate it's not on the road all year round of course).