Diamond Calk Horseshoe Company
The Diamond Calk Horseshoe Company of Duluth, Minnesota, USA was founded in 1908 by blacksmith Otto Swanstrom. Initially manufacturing horseshoes with a special type of calk to improve the animals' foothold on slippery surfaces, the company successfully adapted to the development of motorised transport for the masses and produced a range of adjustable wrenches and pliers from the 1920s. The family-owned company was sold to the Triangle Corporation in 1981; Triangle itself was eventually sold to Cooper Tools.
- Norton, Maryanne C.; Aubut, Sheldon T. (2001-07-30). "Diamond Calk Horseshoe Company". Duluth, Minnesota. Arcadia Publishing. p. 54. ISBN 0-7385-1891-3.