20 C.F.R. § 404.240
If you had all or substantially all your social security earnings before 1951, your primary insurance amount computed under the "1977 simplified old-start" method may be higher than any other primary insurance amount computed for you under any other method for which you are eligible. As explained in § 404.242 , if you reach age 62 after 1978, your primary insurance amount computed under the old-start method is used, for purposes of the guaranteed alternative described in § 404.230 , if the old-start primary insurance amount is higher than the one found under the average-monthly-wage method. We may use a modified computation, as explained in § 404.243 , if you are entitled to a pension based on your employment which was not covered by Social Security.
20 C.F.R. §404.240