(1) A person upon whom a notice of assessment is served may within six years after the end of the year of assessment in which the assessment was made apply in writing to the Director General for a revision of the assessment on the ground that the assessment is excessive by reason of an error or mistake in a return or other statement made by that person for the purposes of the assessment.

(2) No relief shall be given under subsection (1) in respect of an error or mistake as to the basis on which the liability of the person concerned ought to have been computed where the return containing the error or mistake was in fact made on the basis of or in accordance with the practice of the Director General generally prevailing at the time when the return was made.

(3) An application under subsection (1) shall be, as nearly as may be, in the same form as a notice of appeal under section 18 (indicating as the grounds of appeal the error or mistake relied on), and shall be deemed to be, and shall be disposed of as if it were, an appeal under that section, the application itself being taken as the notice of appeal.