The Brooke era lasted from 1841-1941 in Sarawak. The Japanese occupied Sarawak during World War II from 1941-1945. At the end of 1945, Sarawak was nominally an independent country under the Brooke dynasty.
Vyner Brooke formally ceded sovereignty to the British Crown on July 1, 1946, following a controversial vote in Sarawak's Council Negri (legislature). Sarawak became a British colony (it was formerly an independent state under British protection).
His nephew, Anthony Brooke, continued to claim sovereignty as Rajah of Sarawak. (Source: 1) He was District Officer of Sarikei in 1941. He was linked to the anti-cession movement. (Cession means ceding of territory to another country by treaty.)
Vyner Brooke, in 1941, promised Sarawak independence but then handed over Sarawak to be a colony in 1946. This was what started the anti-cession movement (source 3). Protests happened from 1946-1949, after which the British Government closed all legitimate means of reversing this change in constitutional status.
The cession of Sarawak produced a split in the Malay community. In 1949, some Malays who opposed the cession assassinated the first British Governor of Sarawak, Duncan George Stewart. (Source: 2). Duncan was been welcomed by a line of school children during his first official visit to Sibu on 3 Dec 1949 when he was assassinated. (Source: 4)
John Pike, a Magistrate, 3rd Class, acted as the defense counsel for the accused. (John Pike succeeded Alastair Morrison as Assistant District Officer of Sarikei District in early 1949). Pike pointed out that the accused was young (18) and impressionable and also helped the British Crown to uncover the Rukun Tiga-belas movement, the secret anti-cession group behind the assassination. He lost the case and the accused was sentenced to death by hanging. (Source: 4)
In 1951, Anthony Brooke, fearing that he would be branded as a traitor to Britain, sent a message to anti-cession groups to urge them to give up the fight against British rule. The anti cession movement did not end because of this but gradually died after this. (Source: 3)
Now you know why those old stamps you collected in Sarikei had Queen Elizabeth's images from the British Crown colony days.
(1) Wikipedia (Sarawak)
(2) Malaysia--new States in a New Nation: Political Development of Sarawak and Sabah in Malaysia. By Kanagaratnam Jeya Ratnam, R. S. (Robert Stephen) Milne
(3) South East Asia, Colonial History. By Paul H. Kratoska
(4) From British military intelligence to financial secretary of Sarawak: John Pike 1945-1967.
(note: John Pike succeeded Alastair Morrison as Assistant District Officer of Sarikei District in early 1949. In late 1949, Pike was posted to the Binatang as Assistant District Officer. In 1951 Pike was appointed District Officer to the Lawas District. Pike became the Sarikei District Officer in 10 July 1954 and that's where he encountered the 1955 trade hartal started in Sarikei. )