"opener" as comparative adj.

[Alison Murie]

 From Todd's _Birds of Western Pennsylvania_:  ...."crossed Dutch Run
a second time, and shortly thereafter entered opener country in
Indiana County."
Whether these words were written in 1940, when the book came out, or
in 1893 when the field notes for this day's events  were probably
written, isn't entirely clear from the text.
Can't remember ever coming across this usage before.
AM

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