It may be a private plate as mentiond, you can only have a private plate that makes the car look older than it is not the other way around i.e A Sreg car could have A15 ABC but not X69 ABC.
As said if its a Q plate it has had parts off another car. There are ways around getting a Q plate, if your car has a certain % of original parts from each area of the car such as suspenion, engine etc then you can retain the Identity.
If you were to write your Mk2 Mexico Escort off and wanted to rebuild it, this is how you keep it as a Mexico model so it doesnt loose its value or characteristics.
If your unsure about how old your car his try a search on Parkers or the auto-trader. Mazda bought the rights and the machinery to produce the 121 based on the Fiesta Mk4, so the chances are they started to make them AFTER Ford was done with them so your talking about 1997/98
To me that indicates its got a privateplate, that and the fact that in 93/93 (Lreg) Ford hadnt created the Mk4, so Madza couldnt have bought the rights yet!