McIver Post Office was in Rockingham County, North Carolina, not Caswell County, North Carolina, but it served Caswell County residents, as the following will explain. It was located on the present NC 150/NC 87 about a mile east of Thompsonville (now Williamsburg) in Rockingham County. McIver was the successor to several other rural post offices along the Caswell/Rockingham border. For example, Lenox Castle (1799) and High Rock (1814) in Rockingham and Ashland (1878), Eastland (1887), and Gannaway (1887) in Caswell, eventually gave way to Thompsonville.
However, because the name Thompsonville was too often confused with that of the larger Thomasville, the postal name for the former Thompsonville was changed to McIver in 1907. McIver, too, was rather short-lived, for soon rural free delivery enabled the area to be served by the larger Reidsville post office. Some Caswell folk on NC 150 may still be served by a Reidsville rural route, just as Kill Quick, Caswell County, is on a Ruffin rural route. At one time Joseph L. Waynick (and later his daughter Maggie Burton) was postmaster.
Remember that "post offices" were peripatetic, often just a corner of a postmaster's living room, or perhaps behind the counter of a country store, and they sometimes were discontinued and replaced by another a few miles up the road, often of a different name. County boundaries made little difference to the post office department in Washington where patronage meant that the politics of the postmaster was what mattered.
Source: Dr. H. G. Jones.