The Army Post Office was incredibly efficient and letters posted by soldiers were checked and sent across the channel in a few days. The envelopes of these letters were stamped with post marks indicating that they had been censored and been through the postal system. Recently I have picked up about sixty different World War One active service envelopes that show the diversity of these post marks:
These stamps come in different shapes and colours, we have triangular examples:
And sqaure and rectangular stamps:
From these envelopes, there is a large enough sample to draw some basic conclusions. The actual stamp for the Field Post Office is consistently stamped in black and is circular in design, with the date in the centre and the divisional number beneath. Censorship stamps come in a variety of shapes and tend to be stamped with red, purple or blue ink and have a crown, together with the number of the censor. These changed according to date. Very early letters were stamped with a circular stamp, however by November 1914 this had been replaced with the square stamp which lasted until April 1915 when a triangular stamp was introduced. The hexagonal stamp came into service in January 1916, the oval design in November 1916 and finally in October 1917 the rectangular stamp.
These stamps were issued in huge numbers to officers censoring mail, sometimes sequentially and sometimes in random order for security purposes.