Inverted center error stamps are of course rarities because that is the major error in the production of the stamps. There are many different forgeries about this inverted center 5 Mk Viking.
The stamp above is genuine and its trade mark is: FRAME OFF CENTERED. The length of the black frame (outside measure) is 29.50 mm. Source: Pertti Hujala, Finland.
The length of the black frame below is 30.23 mm. That is a forgery, printed on quite white paper and it is "too well centered". There are many other forgeries about this inverted center Viking 5 Mk. The white paper forgeries eventually belong to the early production. If you are not careful, you will be deceived easily with this quality forgery.