I actually mentioned that scene in a draft of the entry. But I cut it as it doesn’t seem to fit the usage pattern. If someone was using the phrase slim customer in the early twentieth century, it would most likely be a South African, not an American. While it is certainly possible for an American to have used the phrase, it seems more likely to be the invention of a Hollywood screenwriter than a case of actual usage.
Still, it is a good scene. (Despite the problematic nature of casting Alec Guinness as an Arab.)