The phrase "earliest reference" means the first recorded or mentioned instance of something. It refers to the earliest point in time when something is mentioned or acknowledged. Full definition
The headlines mentioned earlier reference the main takeaway of the report: that $ 74 billion figure as a misstep of the Department of Education. (studentloans.net)
What you can't tell from the images is that a recording was playing in the booth in which Early references coming to terms with his sexuality (joannemattera.blogspot.ru)
The earliest references to it were contained in speeches by the then Governor in August 1992 and March and August 1993. (rba.gov.au)