Showa Seisakusho, Osaka, 1939-60
According to the information on the Japanese Wikipedia page (see this link) Showa was originally a seller of a 2 stroke machine made by Meguro in 1939 and then reformed as an independant manufacturer post WWII. Starting with a series of small 4 stroke machines in 100cc and 150cc displacements, in 1956 they switched over to 2 stroke engines which appear to have been heavily influenced by East German MZ designs. By 1958 they had introduced oil injection on their engines some 6 years before other Japanese manufacturers were to do so. In 1959 they absorbed the Japanese High Speed Agency (manufacturers of the HOSK line of 4 stroke machines) and then in turn were themselves absorbed by Yamaha in 1960.
Showa produced models under the Changwa (昌和号), Light Cruiser 125cc (ライトクルーザー号), Cruiser 250cc (クルーザー号) and Marine 350cc (マリーン号) brands.