American and United and Eastern had uniforms that were similar to BA. I think Pan Am looked like that too. The airlines were flying a lot of turbo props in the early 1960s.
Now that we have an idea of the type of airplane (and the stewardess) most likely taken on a flight from Kenya, now we need the passenger lists for flights during the Summer of 1961. Probably in some archive deep in the bowels of Interpol. Probably impossible.