Cash box issues are freely accessible here.
This archive has high resolution documents. https://digitalarchive.wm.edu/
This source has mixed resolution documents. https://archive.org/details/cashbox
From Wikipedia: Cashbox, also known as Cash Box, was a music industry trade magazine, originally published weekly from July 1942 to November 1996.
Those are a great source of info for diving into U.S. recording history.
Cash Box magazine
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