My view is that the country of production should define a piece's "nationality"
In the case of Sam Herman, who was born in Mexico, but worked in USA, moved to the UK, then Australia, then back to the UK and is now a British national, glass made in USA would be American, glass made in Australia would be Australian and glass made in the UK would be British.
When a French vintner moves to South Africa, we classify the wine as South African....