Another source to use would be Jodeen Popp's book, "Competitive Drum Corps ... There and
Then to Here and Now." It's not a very scholarly book, but it has lots of entertaining anecdotes about corps up to the late 1970s. Also, to address another criticism, "she was there."
Good luck finding it, though. When I last ran a search on it, back in 1995 or 1996, fewer than a dozen libraries had copies.