Someone else linked another version of the story in another comment, which answe...

Someone else linked another version of the story in another comment, which answers it: https://www.panam.org/pan-am-inspirations/634-saga-of-the-pa...

A banker in Australia advanced them $500 USD, which was enough to finance the rest of the journey.

I believe that would be about $9k USD adjusting for inflation, which feels oddly low given the length of the flight, but I suppose they wouldn't have needed to pay for fuel or supplies at PanAm stations.