Notice deemed served when assessee appears or cooperates, barring objections unless raised before assessment or reassessment completion. The new Wealth tax provision deems notices duly served where the assessee has appeared in proceedings or cooperated in an inquiry relating to assessment or reassessment, precluding objections that the notice was not served, not timely served, or improperly served; the bar does not apply where the assessee raises such objection before completion of the assessment or reassessment.