You may already have checked this - in the Canadian Civil Aircraft Registration database
http://wwwapps2.tc.gc.ca/Saf-Sec-Sur...en/history.asp CF-TDL does not show any records far enough back to be useful. My thoughts for type are that they are unlikely to have been using DC-3 for oceanic flight. and DC-6 first flew 1946 so I think DC-4 or C-54 is most likely. U.S. FAA registration database is at
http://registry.faa.gov/aircraftinqu...t_inquiry.aspx but on first glance only gives curent information, not historical.
I don't know enough about Customs rules to define "exempt" but wonder if they are people who would currently be classed "diplomatic".