171. Assessments or amended assessments to be final

Where no valid appeal has been lodged within the time specified in subsection (1) of section 165 against an assessment as regards the amount of the assessable income assessed thereby, or where an appeal preferred against such an assessment is dismissed under subsection (9) of section 165, or where agreement is reached under subsection (7) of section 165 as to the amount of such assessable income, or where the amount of such assessable income has been determined on appeal, the assessment as made or agreed to or determined on appeal, as the case may be, shall be final and conclusive for all purposes of this Act, as regards the amount of such assessable income :

Provided that nothing in this section shall be read and construed as preventing an Assessor from making, subject to the provisions of section 163, an assessment, or additional assessment for any year of assessment, which does not involve re-opening any matter which has been determined on appeal for that year.