The 27.0mm post is curious. Tange No.2 uses a seat tube that is actually double butted and has a top butt od 0.9mm. For an imperial diameter tube, this makes the inner diameter, 26.8mm, which would typically uses a 26.6mm seat post. For it to use a 27.0mm post, it would have to be custom or reamed quite a bit. One brand that used a custom No.2 tube with a 27.0mm post during this era was Centurion. Their 600EX model in 1984 was the Comp TA and while it was availble in this colour of read, it typically had a contrasting head tube and wasn't manufactured by Miki.