Mount Gay Extra Old or XO as they seem to have re-branded it. For those in the know Mount Gay Extra Old is often known as MGXO. The XO notation on the Mount Gay is pretty standard across the rum and spirits industry to denote an aged spirit (without actually noting how old it is!).
As the Mount Gay Distillery is the oldest official distillery in the world established 1703 as the label denotes, I don’t think we need to worry too much about the heritage and authenticity of the contents of this Extra Old rum.