My question is still: When did Social Security become the national
retirement act?

1935

From SSA.gov FAQ;
Q4: Is it true that Social Security was originally just a retirement program?

A: Yes. Under the 1935 law, Social Security only paid retirement benefits to
the primary worker. A 1939 change in the law added survivors benefits and
benefits for the retiree's spouse and children. In 1956 disability benefits
were added.