Character mistake: Hicox uses the word "momentarily" in the American sense of "in a moment". In British English it means only "for a moment". An educated Englishman in the 1940s would never have used the word in the American sense.
Continuity mistake: When Shoshanna meets Zoller, there's a strand of hair falling by her left cheek which appears or disappears depending on the angle focusing.
Chosen answer: It is explained shortly afterwards that a real German would hold up his thumb, index and middle fingers to indicate three. Since, the major already suspects the Captain of being a spy, holding up his index, middle and ring fingers to indicate three confirms this.
Xofer