Ah thank you for the extra photos, yes those covered buttons with O ring metal backs and fabric shanks are definetly earlier (they are the same sort I have on a 40s blouse I posted in yesterday if you want to look) I agree a 30s dickey is a possiblity. I have some satin ones I may be able to find for comparison. They don't look so much like a waistcoat as this one does though, more like blouse ruffles.
I am still curious about the size - an EU 42 is around a UK 14 in my experience. I wear a 38 as a UK 8/12 so this still leads me to wonder if it has been altered in the past. Can you compare the stitching and thread used around the garment to see if there are any anomalies.