Sometimes it takes a little foot work and/or a few dollars. The best place to start is the Historical Society of the area you're interested in. Sometimes a website will produce a lot of useful information. Sometimes a book written by a local that sought the info of the past has a lot of good information. Even talking to some old timers gathers good information.
MineLab E-Trac, MineLab Safari, MineLab Explorer XS,
White's 5000 D, White's 6DB, White's 5000 D GEB
Sunray X-1 for the ML's, Garrett PP for the White's and a Backhoe