Shimazu Motor Research Institute 1908-1926
Japan Motors Manufacturing 1926-1929
I have drawn from several sources for this, but the main one I've relied on is "国産二輪車の誕生から100年、最強の二輪車国となって50年 : 先駆者・島津楢蔵と戦後二輪車のイノベーション" by Tsutomu Demizu of the Osaka Sangyo University. A version of the same material appears in the book 'Japan's Motorcycle Wars by J.W. Alexander and both versions appear to be based on a 1972 interview with Shimazu Narashiro by Hashimoto Shigeharu.
The "Shimazu Motor Research Institute" was founded by Shimazu Narashiro (there are several variations on the spelling of his name) who appears to have been a genuinely remarkable man. He designed what is generally considered to be the first domestic motorcycle in 1908. This used a 400cc 2 stroke engine of his own design. In 1909 he built a 4 stroke engine of 400cc and F head design, and fitted it to a frame also built in house. This he named the N.S. based on his initials. It used a belt drive and some 20 units were sold.
Some years later, after working on aero and auto engines, Narashiro returned to motorcycle design, and in 1926 produced the Aero First which used a 633cc single cylinder, 4 stroke side valve engine putting out 6.5 HP. Six of these were built before he went bankrupt the same year. He then formed a partnership with Yukio Ohbayashi to form 'Japan Motors Manufacturing', refined the original Aero First design and went into commercial production of the new version which had a 250cc, 4 stroke single cylinder engine and a 2 speeed gear box. About 700 of these machines were sold before the company was wound up in either 1927 or 1929.
Narashiro continued on in other engineering projects and was active into his 80's. He died in 1973 and was inducted into the Japan Automotive Hall of Fame.