Hi again,
I think 'say' is a mishearing of 'save', as in the phrase 'save for', meaning 'except for'.
His maps were much like our own, save for one Alonpore exception, the undiscovered continent of America was entirely absent.
'Alonpore' is probably some other word that was not heard correctly, too. eg anomalous? obvious? I don't know.
Welcome to the Forum, Molly.
Best wishes, Clive