12. Where, following the amendment made under the former Corporation Tax Act in computing depreciation, a corporation has a taxable difference for a taxation year beginning after 1961, such corporation shall, to the extent and on the prescribed conditions, add in computing its taxable income, each taxation year, all or part of such difference, to the extent that it has not yet been added in computing its income for the years after 1961, according to the rules hereinafter determined:(a) if the taxable difference does not exceed $1,000, the total of such difference;
(b) if the difference exceeds $1,000, a part of such difference, computed according to the method of residual value, by using the rate for the class of property to which the origin of the difference may reasonably be attributed; or
(c) if the residual value of the taxable difference is less than $1,000, the total of such residual value must be added to the income.