... the letters however always match on one side of the tile to the other. (ie if it's Dx on one side then the other side of the tile will be Dy)...
Just went and checked, and apparently one of the tiles is 7B/5F. Oops. So the pattern works most of the time, but not all the time. (this is made weirder by the fact that since there is no other tile lettered F that they could easily have made it 7F/5F, but oh well...)
On the other hand, that tile has the only F and the only room surrounded by road on all side, so it's unique in that way as well making it easy enough to remember and identify.