Assuming it was a Miyata manufactured frame, this would be a 1986 model manufactured during the last fortnight of 1985. At that time there were no aluminum frames in the either Miyata's North American or Eurpoean catalogs. In fact, Miyata's first aluminum frame (late 1986 model) was welded. The bonded frames were introduced even later. I'd appreciate a good photo of the seat lug.
In the very late 1980s, Univega had bonded frames with aluminum main tubes and steel rear traingles, They were not made by Miyata, but a Taiwanese company.