The installation is to be found on a few aircraft of the period, the Boeing 307 springs to mind. After writing the previous sentence a search produced the below photo which had the caption "Pan Am Boeing S-307 Stratoliner
NC19902 Clipper Rainbow with blanking plates installed on engines to prevent engine over-cooling.TWA aircraft were fitted with cowl flaps, to adjust engine cooling air, while Pan Am aircraft had fixed cowling rings without cowling gills. When operating in cooler conditions, the Pan Am aircraft could be fitted with a blanking disk that covered part of the front of the engine". I think the answer has been found. Thank you all.
