I am willing to bet that Ford never built a one-off three door in 1991. The economics just don't add up for them to either retool a line that was scrapped when the Mk2 came along, or to put a team in a warehouse and hand build the car from parts. A dealership, enthusiast or bodyshop might have done that though, or it could be a reshelled sapphire into a new 3 door shell. My XR4i shell used the parts from a J reg XR4x4 and should get a J plate.

Or it might just be a late registered car. Remember that a car can be built and sold without being registered. Then, maybe years later, when you decide to put it on the road,you need to get it registered. The number plate is decided by the date of first registration.