Passage: Basu was elected to the Bengal Legislative Assembly in 1946, contesting the Railway constituency. He served as the Leader of Opposition for a long time when Dr. Bidhan Chandra Roy was the Chief Minister of West Bengal. Jyoti Basu led a number of agitations against the State Government and earned enviable popularity as a politician particularly among the students and youth. Beside organising the movements of the Railway Labourers, he led a movement by the teachers demanding a hike in salary. When the Communist Party of India split in 1964, Basu became one of the first nine members of the Politburo of the newly formed Communist Party of India (Marxist). In 1967 and 1969, Basu became Deputy Chief Minister of West Bengal in the United Front governments. In 1967, after the defeat of the Congress Government, Jyoti Basu was sworn-in as the Deputy Chief Minister under the Chief Ministership of Ajoy Mukherjee. In 1970, he narrowly escaped an assassination attempt at the Patna railway station. Though CPI(M) became the single largest party in the assembly elections in 1971, the party was refused the chance to form a ministry and presidents' Rule was imposed in West Bengal.

Question: when did Basu become a member of Politburo
1964