BIGOT was a World War II security classification at the highest level of security - above Top Secret.
BIGOT stood for the British Invasion of German Occupied Territory and was chosen by Churchill before America came into the War and it remained the security classification even when Eisenhower took over the planning role.
Everyone with knowledge of the D-Day planning work - Operation Overlord - which included the landing beaches and the date of the Invasion were security cleared and were listed on what was known as the 'BIGOT list'. Those on the BIGOT list were banned from travelling outside the UK in case they were captured and coerced into talking. There was one exception to this - Churchill himself!
Those on the BIGOT list had knowledge of one of history's biggest secrets and the fact that Overlord was successfully carried out is testimony to the work of the planning team and the fact that the secret was kept.
The stamp was used to mark all Overlord papers and files.