Well, the obvious, photographs. Specifically, photographs of the entry signs. This often includes all access points that have entry signs. Over the decades have seen many of these signs change and then change back to the original. The most interesting story was when the Grand Teton entrance sign (Elk Refuge side) was stolen and a year later found abandoned in a home, in California.
Also, Postcards that I mail from of 407 National Park Properties I have visited, as well as state parks and museums of note (Norman Rockwell as an example). I currently have 22 persons on my Post Card Mailing list.
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