I hate to burst your bubble, but it aint' Horowitz' piano. The instrument in question came to my home town of San Antonio about ten years ago, and I played the Schumann Arabesque on it.
I mentioned this to my tuner, who is a Steinway Factory trained technician, and he told me that this is not the same action that was in the original. Further, he worked on the instrument when he was in New York, and like other technicians before him, he signed it underneath.
The Steinway people were none too pleased. So, I am told that there are about two versions of this thing going around.
Further, Horowitz used to put lacquer on his bass hammers. When you would hit a normal bass chord, it would sound like a bomb going off. This instrument has no lacquer on the hammers.
So, check for the name of J. Verdusco somewhere underneath the piano, and also check for lacquer on the hammers. If these factors are not present, then you are not looking at the real deal.
It won't be the first time Steinway lied. They used to send out a real stiff action concert grand in New York, and if anybody complained about it, they would say that it was the piano that Rubinstein played.
They don't do that anymore because they got busted for lying about it more than once. Earl Wild relates in his memoir how they tried to pull this on him, and he called them on it, and they backed down.