In 1971, President Richard Nixon signed legislation providing a 10% increase in Social Security benefits (and a hike in the payroll tax rates, from 5% of the first $7,800 in wages to 5.15% of the first $9,000).  A retiree who would have received $500 a month at the beginning of 1969 would be receiving $759 per month, or 51.8% more, by the end of 1972.

Source: Sylvester J. Schieber and John B. Shoven, The Real Deal: The History and Future of Social Security (Yale University Press, New Haven, 1999), pp. 158-159. 161.