I've worked and owned Miyatas; this could well be a Miyata. Koga Miyata is how the bike was branded in Europe, not the States. Usually when there is a question about the provenance of a frame, it is because the bike has been restored with decals that make it seem like a better bike. That is not really the case here. The manganese frame tubing (I think that is what the tubing decal says) came on some Miyatas but it was not their high end tubing. The bike looks great. Personally, I'd ride it and not worry about whether it is a Miyata or not.