I took a lot of convincing charliebrown, but with advances in home computing it is now much easier to manipulate the original image and reveal its true identity than it was back at the start of this thread and indeed this forum (and also the days of 'The Unexplained' magazine and 'Strange but True").
It is his wife and as already pointed out, you can even see her bra strap. It has been a wonderful mystery to pursue over the years and it gives me a warm glow on this gloomy Sunday afternoon to visit this old familiar thread again, but it has been explained. Here is Mum's dress again:
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Now look at Ermintruder's post above - they are identical sleeveless garments