Short answers, pulled from the story.
Mainichi Broadcasting System began television broadcasting at 10 a.m. on the 1st of March 1959. The station had originally planned to launch on the 1st of December 1958 but pushed the date back after losing access to programming from its intended Tokyo partner.
Mainichi Broadcasting System switched networks after Japan's national newspapers traded their television station shares in the early 1970s. TBS then invited Mainichi Broadcasting to join its network in the summer of 1974, and the two companies announced the move on the 19th of November 1974, effective the 1st of April 1975.
Mainichi Broadcasting System's current television callsign is JOOY-DTV. The callsign changed from JOOR-DTV on the 1st of April 2021, when the company's radio operations were spun off into a separate business, MBS Radio Co., Ltd.
Mainichi Broadcasting System is headquartered in Chayamachi, in the Kita-ku ward of Osaka. Its main building was completed on the 1st of September 1990, on the former site of the Hankyu Department Store Distribution Center, and an expansion, B Building, was completed on the 4th of September 2013.
Mainichi Broadcasting System partnered with Nippon Broadcasting System to establish FM802, serving as a major shareholder in the new station. Nippon Broadcasting System organized the station at its founding, while FM802's first sales manager was seconded from Mainichi Broadcasting System.
Mainichi Broadcasting System was the fourth television station to launch in Osaka, despite being the oldest station in Japan to begin radio broadcasting. It launched roughly the same time as Asahi Broadcasting Television, about a month behind.