Thanks for your help!
Unfortunately I didn't receive any old registrations or export documents with the car.
It was bought through a Dutch dealer and passed through one other set of hands in Canada
before I got the car so any paperwork has been lost.
It just occurred to me that there was a service sticker from a Mazda dealer in Rotterdam, but it was not dated.
The car had 115,000km when I got it so the sticker is quite old.
I took a close look at the Umwelt decal (and a closer picture)
The writing is definitely 75-PF-NI
I checked the link you sent and it looks like the numbering could match the information for Dutch number plates (sidecode 6) but any additional information is appreciated.