Japanese rivel manufacturer Miyata started manufacturing frames in 1890. Miyata grew big in the US during the 80s. They are widely considered the best 'of the shelf' bikes of that era. By mid 90s Miyata disappeared from US market because of expensive Yen and cheap production in Taiwan. Same happened to makes like Panasonic and Bridgestone. In Japan tey are still present with a to us quirky line up of bicycles.
Koga Miyata was founded in 1974 in Heerenveen, Friesland, The Netherlands by Andries Gaastra, (coincidentally?) grandson of Batavus founder. Andries son is the man behind idworx. A cycle familly. Koga is a substraction of Gaastra and the family name of his wife (Polish, I do not remember). From the beginning aimed at the luxury market. Andries decided to go with Miyata frames because they offered what he wanted for his brand. In the beginning Koga was Shimano importer as well, so an 99% Japanese package was offered and not much Campa on the high end Capri Sonne roadbikes. Koga were the 1st to bring Giant on the market. ''Giant, quality touched by Koga'', it was. That was before Giant grew big. To my information 1995 was the last year for Japan made Miyata frames at Koga (maybe because of the same reasons why Miyata disappeared from the US market???)
I think that is about it. For us, as customer, it means that Miyata and Koga Miyata frames mainly differ in graphics and paint.