Thanks for Your responses.
First, I'm aware of the fact, that the blade was altered. Because the lower hole is the original, accompanied by some higaki yasurime in the lower third of nakago, I think, the original length must be about 20 cm. The blade must have been broader, too.
Why not Mino? Well, the existence of dark steel, the nie-deki hamon without any togari (although a great amount of activities), pointed boshi, bo-utsuri - all these speak in my opinion against Mino. I assumed this blade may be Sue-Naminohira (ok, the bo-utsuri doesn't fit). However - these are my personal considerations.
Finally, I do not doubt NBTHK-papers in general but in this special case I think because of the dating there's someone quick at hand with a Sue-Mino-attribution. Further studies are needed...
Florian B.