Plates from Mauritania are owner provided and therefore are easily obtained as unused, mint made-to-order specials that have never been on a car. There are more of these made-to-order specials in collections than there are genuine used plates from the current series.
The problem is that some people have them made up with impossible registrations. These are two examples where the region number (20) is one that does not exist. Even though one of them looks like it could be genuine, they are not.
Always be very suspicious of current series Mauritania plates. Ask for photos of the back to check for use. Join a plate society and learn the details of genuine plates.