Section 4(a) of the Act provides that in the treatment of dealer reserve income by the taxpayer is changed to a method proper under the accrual method of accounting, then the taxpayer may elect to have such change treated as not a change in method of accounting to which the provisions of section 481 of the Internal Revenue Code of 1954 apply. This election shall apply to all taxable years ending on or before June 22, 1959, for which the assessment of any deficiency, or for which refund or credit of any overpayment, was not prevented on June 21, 1959, by the operation of any law or rule of law. This election may be made only if the alternative election under section 3(a) of the Act has not been exercised. If an election is made under section 4(a) of the Act, taxable income (or net income in the case of a taxable year to which the Internal Revenue Code of 1939 applies) shall be recomputed under a proper method of accounting for dealer reserve income for each taxable year to which the election applies, without regard to section 481.