I suspect only a real expert on these can tell you whether it is really a Nakayama, and even then there will probably be a decent margin of error. Unless the stamps are on the sides, and even then...

Looks to me as though it definitely is predominantly asagi, with some renge (red pear-like spotting) and perhaps even blending into karasu. As long as it is not too scratchy (a common problem with asagi) it will most likely be a very nice stone, and relatively quick cutting particularly with slurry. It's own slurry, made by a diamond plate (preferably something that is not going to scratch up the surface, like an Atoma 1200 or similar).

Enjoy the stone.

James.