The Atlas HO Scale Master 3 Bay Cylindrical Hopper Car Dresser Minerals (NAHX) 51866 models the 3 bay cylindrical hopper car introduced by ACF Industries in 1961. ACF designed this car to transport sand, clay, plastics, salt, grain, and other bulk commodities. Between 1961 and 1966 about 4,000 units of this type car were built. The 6 bay cylindrical hopper car is important in railcar history as it was the first commercially successful tank-type covered hopper car design. While most of these hopper cars have been retired over the years, a few are still operating on North American railroads. ACF Industries, known as American Car and Foundry until 1954, was formed in 1899 by the consolidation of 13 railcar manufacturers. ACF remains one of the largest manufacturers and lessors of railroad freight cars in North America.
Dresser Minerals: Dresser began in 1880 producing products for the new oil industry then based in Western Pennsylvania. An 1885 technical innovation flexible coupling enabled pipelines to be built without leakage, which spurred Dresser to become a leader in the fields of technology, energy, and natural resources. The company became a publicly traded company in 1928, and continued to grow its business until its 1998 merger with its rival Halliburton. I could find no information specific to Dresser Minerals on the Internet.