In the 19th century Wisconsin became an important mining area of the US. It had important connections to Britain and Ireland as thousands emigrated there from Cornwall, Wales, the north-east and Ireland. Mainly to work the zinc and lead mines. Mineral point was the centre of the Cornish community. In 1860 a small zinc furnace was erected there. It changed hands a number of times but eventually it merged with the New Jersey Zinc Company and it became the largest zinc outfit in the US.
To serve the mines in the area The Mineral Point Northern Mineral Railway was built and started operations on June 17th, 1857. It shipped ore into Mineral Point from Linden, Highland and Mifflin. It was boom time. The railway closed in 1928 followed closely by the zinc furnace crippling many of the mines.
The GR is for Mineral Point.
Just a note. The MP railway depot, now a museum, was built in 1856 by two Cornish stonemasons John Toay and Philip Allen.