Undivided Assam

Undivided Assam refers to the then undivided greater state of Assam in India soon after the Indian Independence in 1947 till 1963. In addition to the present-day Assam, it included the current states of Arunachal Pradesh, Meghalaya, Nagaland and Mizoram. The capital of this state was Shillong, currently the capital of Meghalaya.

Prior to the Partition of India, Sylhet was a part of Assam.

...
Wikipedia