Aria, Aria Diamond, Aria Pro II are brands of the Arai and Company firm, created in 1953 by Shiro Arai in Japan (also see infos on Arai brand). Their guitars were initially labelled Arai, then switched to Aria around 1966.
Aria established a very close partnership with Matsumoku (Aria has been Matsumoku's main "client" for about 22 years) who built Aria guitars from 1964 to 1987 when Matsumoku closed down. During this period, Aria also had two plants, very close to Matsumoku ones and sharing facilities, resources and equipments.
Aria Diamond was the brand used for early hollow-bodies.
From 1975 onward, after design engineer Nobuaki Hayashi joined Aria (Hayashi's pseudonym was H. Noble, a mention that appears on some instruments he specifically designed), all guitars were labeled Aria Pro II.