"I believe though it is just his first and middle name and we have nothing else to go on"
I´m pretty sure in your case, Antonio, was the last name. From my experience, back in the 18th century Portugal, in many parts of the country, surnames were not given, it is common to see in birth registrations the names of the father, such as, Joao Manuel; Jose Antonio;etc., and Manuel and Antonio would be the surname and would be given from generation to generation. There are thousand´s of examples in Portuguese birth registrations, of "Antonio", being the last name.
Fernando from Portugal email@example.com