In 1981, Bally merged its pinball division with Midway to form the Bally/Midway Manufacturing division. Three games released that year, including Satan's Hollow, were the first to feature the Bally/Midway brand. From the late 1970s through the late 1980s, Midway was the leading producer of arcade video games in the United States.
Source: Wikipedia, "Midway Games," available under the CC-BY-SA License.