Nice Miyata!, It looks NOS! You should have a lot of enjoyable riding years with that bike!
Regarding the brake cable routing on the bars, unless there are grooves on the handlebars for them, the position of the brake cables under the bar tape is totally up to what the rider prefers. I always put them behind the bars ever since I started using aero levers on my bikes back in the mid 80's, I've always done it that way on my bikes since because I just like the way they feel to my hands as they end up providing a comfortable flattish area on which my palms rest on when I'm on the tops and the clean curve of the bar in front conforms better to my fingers when I grip the bar when routed that way and having the cables come out at the back of the bar slightly minimizes the length and bend that the cables have to do to get to the front caliper and towards the front most cable guide on the TT.

Chombi