I don't think its a Miyata at all....

1. Miyata tended to use fixed rear cable housing stops mounted directly to the seat stays on the 6XX and 1XXX. Not on all but most...
2. Low rider munts. Miyata tended to use flush mount braze-ons not franken posts. Maybe on the old, old ones but not the newer ones
3. BB cable routing. Older ones had over the BB mounting then they switched to plastic guides under the BB