The Malaysia Agreement 1963 (MA63), signed on 9 July 1963 by North Borneo, Sarawak, Singapore, Malaya and the United Kingdom, was not about North Borneo and Sarawak joining the Federation of Malaya as the 12th and 13th states.
MA63 was about North Borneo and Sarawak being Equal Partners of Malaya in the Federation of Malaysia.
Article 1(2) of the Federal Constitution was amended on 13 July 1967 to place Sabah and Sarawak as the 12th and 13th states of the Federation of Malaya (now known as Malaysia but not the Malaysia of MA63).
Article 160 of the Federal Constitution defines “Federation” as that set up by the Federation of Malaya Agreement 1957.
There’s no such Agreement.
The Federation of Malaya was set up by the Federation of Malaya Agreement 1948 and reinforced by the Federation of Malaya Independence Act 1957.
The Federation of Malaya was the successor state to the Malayan Union 1946.
Before the Malayan Union, the peninsula consisted of Federated Malay States, Unfederated Malay States and the Straits Settlements.- Sarawak Report