I couldn't read it either. But you can just see the dates it passed and went into effect. The law requiring it is US Code 571.202. Here's the 1971 version of it, which is probably very similar if not identical to the 1969 law. It's quite technical. It begins right at the end of page 1, and goes through most of the second column in page 2.
*Edit* I see it didn't come through at full resolution. If you really want to read it, you can download the pdf here:
https://www.federalregister.gov/citation/36-FR-22902
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