If a school district elects to formally file objections to the findings and conclusions of the Board under section 13 of the act (72 P. S. § 4656.13), the objections shall be filed under oath, in writing, in the office of the State Tax Equalization Board, Post Office Box 1294, Harrisburg, Pennsylvania 17108, on or before October 1 and not thereafter, except upon cause shown and in the discretion of the Board, of the year in which the certification was made in compliance with sections 7(3) and 12 of the act (72 P. S. §§ 4656.7(3) and 4656.12).
61 Pa. Code § 605.21