A stamp is well centred when the impression lies evenly between the perforated edges on all four sides. When a stamp is not well centred it is called "off centre".
The cause of off-centred stamp can be: Perforation not placed at the right spot or Stamp has been cut out of a sheet sloppy.
Official stamps are used by government departments. Often as a check on the amount of mail.
Many countries have issued officials (e.g. Argentine, Great Britain, The Netherlands, The United States of America).
Often these stamps have an overprint stating that fact ("service", "O.S.", "servicio oficial").
Officials of The Netherllands: Armenwet, 1913, International Court of Justice (regarded as Dutch officials):
1934 Cour Permanente de Justice Internationale
1940 Cour Permanente de Justice Internationale
1947 Cour Internationale de Justice
1950 Cour Internationale de Justice
1951 Cour Internationale de Justice
1989 Cour Internationale de Justice
An overprint is an additional printing on a stamp after the stamp was completed. The overprint was not part of the original design. Overprints can be made in words, figures, dots, stripes and images.
There are many reasons for overprints, some of them are:
To change the function of the stamp (i.e. from postage to official).
To add the new name of the country.
To add a commemorative slogan (i.e. philatelic exhibition).
To cover the stamp partially (i.e. king out of grace).
When the face value is changed the overprint is called a surcharge.