Tom from Sweet Maria's likes to reference old source material like William Harrison Ukers "All About Coffee" from the Tea and Coffee Trade Journal Company, 1922. Very interesting historical read and can be read and searched on Google Books. (Use desktop mode on mobile devices to access the search and other features)
Modern usage of light, medium, and dark is baffling and probably comes from consumer packaging and not internal production and roasting communications which may tend to be more precise.
I like to use C1 for first crack to avoid confusion, but that hasn't caught on.
He posted old telegrams to social media a few years ago that showed the coded messages that buyers used to communicate with their companies to prevent competitive spying.