There it is, hidden in plain view (more photos on Great Basin Facebook Page).
That looks to be a somewhat unforgiving area; it is not unthinkable that the owner died near where the rifle was found and his remains were scattered by time and nature. Then again, it would not be the first or last time a hunter simply lost track of where he put his rifle.
Anyway, being as the internet is a fickle friend and often lets superbly interesting news articles vanish, I'm posting two reference links.
Click HERE for the CNN version
Click HERE for the Washington Post version
Below are photos of an intact Winchester 1873 that left the factory in 1891; although it is a bit "younger," it still would be a near twin to the Mystery Rifle.
The photo above shows the ejection port sliding dust-cover OPEN.
The photo above shows ejection port sliding dust-cover CLOSED. It is not unusual to find the dust-covers missing on utility grade 1873 rifles; they frequently jam the action so they are removed and set aside by the owners. The dust-cover on this rifle is not an original. I was unable to see if there was a dust-cover on the Mystery Rifle.