I had a look on the Ask Phil site, just to confirm whether I was right, and I was! From 1879, postal authorities had to distribute copies of their stamps to the members of the Universal Postal Union, the UPU. This was done for identification purposes so that it was known which officially issued stamps were in circulation and could be found on mail from foreign countries. These stamps were given the overprint "specimen". Obviously, only small quantitites of these overprinted stamps were produced and they add spice to any collection! The site does not mention whether this is till done today, or when this practice stopped, but I believe that e.g. Germany still issues stamps with a "Muster" overprint and I know that the Belgian bird definitives are presented to government ministers, with I believe a number printed on the stamp (on the back?).
Here's a specimen overprint in red on a 1937 stamp from Switzerland.
These definitives were overprinted for use of officials of the League of Nations. Nice!