(a) four years commencing on and from the 1st day of April, 1975 or one year from the date of the filing of a return or a revised return under section 14, whichever is later, where the assessment year is an assessment year commencing before that date;

(b) four years from the end of the assessment year in which the gifts were first assessable, or one year from the date of the filing of a return or a revised return under section 14, whichever is later, where the assessment year is an assessment year commencing on or after the 1st day of April, 1975 ,