...Yet this [boundary] line which may be so simply described as "west on the 49th parallel" was not as easily marked on the ground. Perhaps the first attempt to fix its location was made by the Hudson's Bay Company. In the spring of 1823 it abandoned the fort at Pembina on the Red River, taken over from the Northwest Company two years earlier. Astronomical observations had placed the site too far south to be in British territory. A United States surveying expedition soon reached the spot, under Major Stephen H. Long of the Topographical Engineers. His astonomer, after four days' observations for latitude, determined a point for the parallel as it crossed the Red River. On the west bank an oak post was forthwith planted bearing on its north side "G.B." and on the south "U.S." This was the mark that some wag later turned around. Major Long issued a proclamation August 8, 1823, claiming the land south of the post for the United States. Yet neither his action nor that of the Hudson's Bay Company had lasting effect.
...Major Long's post on the Red River was renewed substantially in the same place by Captain John Pope, also of the Topographical Engineers, when he visited Pembina in 1850. Meanwhile the Hudson's Bay Company had in 1845 reinstated its fort or store a quarter of a mile to the north, in a position which later observations by Mr. Nicolay, an American scientist, confirmed as being in British territory. When Capt. John Palliser, the British explorer, visited the Hudson's Bay post in 1857, he "adopted" Nicholay's fix, although his own observations brought the 49th parallel 370 yards farther north. This multiplicity of positions may have inspired the citizens of Pembina to erect in 1860 what became known as the "whiskey post,"1 about a mile north of Long's and Pope's, to stop the smuggling of liquor into the United States from a house near the line. In effect, their action created a no man's land and perhaps opened the door to more serious encroachment ten years later.
From West on the 49th Parallel: Red River to the Rockies 1872-1876, by John E. Parsons
1 - This later post was never taken very seriously as it was put in for a special practical purpose. A man from the Red River settlement built a house close to the "Pope's Marker" and used it as a base to smuggle liquor into Pembina, a location convenient to both exporter and importers but too easily accessible to suit the good people of Pembina. It was to strike a blow at those liquor dealings that the more northerly post was erected and aptly dubbed "Whiskey Post." - From Adams George Archibald, First Lieutenant-Governor of Manitoba