Which authorities were AO originally meant?

Joint commissioner was there earlier also.

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Assessing Officer(AO) means the Income-Tax Officer or Assistant Commissioner of Income-Tax or Deputy Commissioner of Income-Tax or Joint Commissioner of Income-Tax or Additional Commissioner of Income-Tax who is authorized by the Board to exercise or perform all or any of the powers and functions conferred on, or assigned to an AO under the Income tax Act, 1961. ""