No return of income, assessment, notice, summons or other proceeding, furnished or made or issued or taken or purported to have been furnished or made or issued or taken in pursuance of any of the provisions of this Act shall be invalid or shall be deemed to be invalid merely by reason of any mistake, defect or omission in such return of income, assessment, notice, summons or other proceeding if such return of income, assessment, notice, summons or other proceeding is in substance and effect in conformity with or according to the intent and purpose of this Act.