(a) The county commissioners, acting as the board of revision, or board of revision of taxes, or board for the assessment and revision of taxes, as the case may be, in each county, shall, on receiving the returns of the elected or appointed assessors, proceed to examine and inquire whether the same have been made in conformity with the laws of this Commonwealth, and whether all property to be valued for taxation for county purposes has been valued at actual value. They shall receive and consider the written communication of any taxable inhabitant of the county relative to any property which such taxable inhabitant shall believe to have been valued too low, and, on the day appointed for determining whether any property has been valued too low or too high, they shall proceed to raise or lower the price or valuation of any property which they shall believe to have been valued too low or too high, and if they cannot on the day appointed revise, raise and equalize the valuation of all property, they may adjourn from day to day until the whole of such valuation shall have been revised, raised or equalized.

(b) The board is authorized to make additions and revisions to the assessment roll of persons and property subject to local taxation at any time in the year, so long as the notice provisions are complied with. All additions and revisions shall be a supplement to the assessment roll for levy and collection of taxes for the tax year for which the assessment roll was originally prepared, in addition to being added to the assessment roll for the following calendar or fiscal tax years.