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Sounds like another surname. Although Söderbäck is a locality in Sweden it's just Söderbäck farm, which seems more likely to be named for the family rather than the other way round. While it just means "South Beck" (a beck is another word for a brook or stream), the particular beck in question is not officially known by that name, and only has meaning in relation to a local frame of reference. In your case the Finnish term seems comparable to "Soderbacka (nee Pyntarvisti)". It would seem both names are Finnish adaptations of Swedish names.