Hi Antibeous.
Like McDave said, the library is a good start. Get hold of copies the of the local paper for as far back as they can go. It might be on micro fish. By looking through there, you might find some clues as to where festivals and celebrations were held in those days, plus some clues out of left field that you may not have even thought about. Charles Garrett has written some good books on how to think when it comes to hunting old spots. Well worth the read. I live in Australia, but I have seen guys post here from time to time, about getting using your local City Hall for info ie old maps and the names of property owners of spots you may want to hunt. I have seen links to websites that are dedicated "old map" sites that will help you locate old ghost towns etc posted on a couple of the Findmall forums, from time to time. Typing in 'old maps' in the search engine above, might help you to find those links.
BTW. That washer you found, reminded me of a find that looked just like it, that was made only a few weeks ago by an Aussie, only 2 hours down the road from where I live. The guy found what he thought was a washer (that had been hit by a pick from another detectorist) and put it in his finds bag so that he could take a better look at it when he got home. His wife suggested that he throw the 'washer away' as it was only another piece of junk. On cleaning up the washer, he found that it was in fact a Holey Dollar, with a guesstimate worth of US $250 000!!!!! Not a bad piece of junk! I heard about it from a chap who has seen the coin in question. He sold the detector (an X-Terra705) to the chap that made the find. The coin is in a bank volt somewhere. Moral of the story, if in doubt, don't throw it out.
Some folk find that doing the research for new sites has it's own excitement and enjoyment that they find as enjoyable, as the detecting part. It becomes, part of the chase. It creates good luck. After all, good luck is only 'opportunity with preparation'.
Mick Evans.