Premise: Keystone is an unincorporated community and census-designated place in central Keith County, Nebraska, United States. It lies along local roads near the North Platte River, northeast of the city of Ogallala, the county seat of Keith County. Its elevation is 3,100 feet (945 m). Although Keystone is unincorporated, it has a post office, with the ZIP code of 69144.
Hypothesis: Keystone's post office operates only during weekdays because this unincorporated community has elevation of 945 m.