I read your 1960's study in more detail.   As no surprise, at age 9, the control group and preschool group had the exact same IQ level of 87.    What you also FAILED to mention is this was not just preschool for needy kids.   This was part time preschool augmented by family intervention and the participants were often single parent African American households.    The cost savings cited in the study were questionable and underlying factors had not been considered in totality.   Here's a link of an assessment done in 2011.

 http://www.welfareacademy.org/pubs/early_education/pdfs/Besharov_ECE%20assessments_Perry_Preschool.pdf