Thanks for the clarification on the "why". Can you confirm "when" were to enlisted crew members promoted to sergeant? I'm guessing when they completed training and sent overseas or when the crews were put together for their final training before deployment.
I have pictures of my father wearing wings but with only one stripe on his sleeves. He said he was a gunner but his discharge listed him a air frame mechanic. Also he was discharged when his group was at Kearns Airfield in Utah which I think was the one of the training sites before bomber air crews were deployed.