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I think the white feather in the wing may cause some confusion but this is too far forward for the speculum which is in any case hidden in this shot, white in Gadwall and as you say blue / purple in Mallard.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
Lat/Lng: 52.034, 0.7348
OS grid ref: TL876408