SRA stands for Science Research Associates, which was a Chicago-based publisher of educational products from 1938 through the early 2000s, when McGraw-Hill purchased them.
Their most well-known product was the SRA Reading Laboratory, a series of grade-specific of reading stories and associated comprehension check questions.
Each series of stories came in a large box targeted at a specific elementary school grade.
Each series was divided into several reading levels, color-coded to allow students to progress to more advanced readers over the course of a school year.
Depending on your starting proficiency, the SRA reading levels were apt to engender various levels of frustration.