For purposes of sections
414
(a) and
415
(d) of this title, an individual shall be deemed to have one quarter of coverage for each $400 of his total wages prior to 1951 (as defined in section
415
(d)(1)(C) of this title), except where such individual is not a fully insured individual on the basis of the number of quarters of coverage so derived plus the number of quarters of coverage derived from the wages and self-employment income credited to such individual for periods after 1950.