Here are some First Day Covers from Sarawak:
06 November 1945: Australian Stamps for Use in Sarawak
These 4 Australian stamps were released for use on 06 November 1945 just after the Second World War.
Most philatelic covers were cancelled on 08 November 1945 when the Australian stamps were widely available.
17 December 1945: BMA Overprint Definitive
The FDC below is cancelled in Brunei.
18 May 1946: Sarawak Centenary
16 April 1947: Sir Charles Vyner Brooke (cypher overprint) Definitive
10 October 1949: 75th Anniversary of Universal Postal Union
03 January 1950: H. M. King George VI Definitive
01 February 1952: H. M. King George VI Definitive
03 June 1953: Coronation of H. M. Queen Elizabeth II
01 June 1955: Queen Elizabeth II Definitive
01 October 1957: Queen Elizabeth II Definitive