Ownership of physical property is also an artificial grant from the state. (Or if you will, a recognition by the state of what people in general believe) Perhaps not a table or a knife, but a farm or a factory, have in many countries been suddenly disqualified as legitimate property of their (former) owner, as a result of e.g. a communist revolution. There's nothing more "natural" to owning a piece of land, than to owning a song.
I'm pretty sure physical possession was not generally considered equivalent to ownership before the 1970s, that's an absurd statement. Shareholders of the East India Company in the 1600s weren't in physical possession of the ships, yet they were considered owners. Even purely intellectual property, such as patents, have existed in laws since at least 1474. Albert Einstein famously worked in a patent office.