Spinners on the props only appeared on some of the earliest delivered DST. They were on Wright powered DST's of American and Eastern and P&W powered DST-A's of United. The only thing I can discern (pure speculation here) is that these early deliveries had counter weighted "controllable pitch" propellers and when "constant speed" propellers were installed, the spinners disappeared (though I do see pictures of ships with counter weighted "controllable pitch props without spinners.
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NC16017 American Airlines Flagship Connecticut (3).jpg [ 200.02 KiB | Viewed 1247 times ]
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NC16086 United Airlines Mainliner.jpg [ 352.6 KiB | Viewed 1247 times ]
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