6. The following amendments shall be made in the Sales-tax Act, 1951 (III of 1951), namely:- (1) Throughout the Act, for the words “Central Board of Revenue” the words “National Board of Revenue” shall be substituted and shall be deemed to have been so substituted on the sixteenth day of December, 1971; (2) in section 2, for clause (3a) the following shall be substituted, namely:- “(3a) “Board” means the National Board of Revenue constituted under the National Board of Revenue Order, 1972”; and (3) in section 17, in sub-section (1), the words “in the prescribed form” shall be omitted and shall be deemed to have been so omitted on the 1st day of July, 1971.