It also looks like Horace, or something similar, to me but I can't see anywhere that says Orazio signed himself Horace.
I would still like to know exactly what the full inscription is
It's not by him, that's why.
I was working on that assumption
That's why there are no other examples like this. It's similar to when someone copies the work of an artist who would normally sign his full name. The copier, or " follower " as it's referred to, will sign the Christian name with an initial. So Thomas Gainsborough is signed T Gainsborough.
I expect we'll see more of these in the next few months. I doubt it's worth the estimate.
( this is all going on the partial inscription we have. There is no other figure / sculpture attributed to a " Horace Andreoni " on the internet )