I agree with most of the comments here. The shape of this is similar to the current Black and Rosewood Standard.
Having seen a few Onyx Platinums before, I have never seen one like this. Nor have much older non Platinum Onyx looked like this.
I believe Koetsu have/had fairly recently a non Platinum Onyx available again, but I've never laid my eyes on one.
I am thinking there are 2 possibilities then:
1. It is a non Platinum Onyx (or Onyx "standard") of late production, which I can't verify as I've never seen one.
2. It is not genuine.
Either way, sorry to say I don't believe it is an Onyx Platinum.
Having seen a few Onyx Platinums before, I have never seen one like this. Nor have much older non Platinum Onyx looked like this.
I believe Koetsu have/had fairly recently a non Platinum Onyx available again, but I've never laid my eyes on one.
I am thinking there are 2 possibilities then:
1. It is a non Platinum Onyx (or Onyx "standard") of late production, which I can't verify as I've never seen one.
2. It is not genuine.
Either way, sorry to say I don't believe it is an Onyx Platinum.