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I found an incoming UK passenger list for Robert & Isabella.
R.M.S. Carinthia, from Montreal to Greenock Scotland, arriving 17 May 1957
Sowersby, Robert, born 2 Nov. 1884, married, address in UK - 12 Redbraes Place, Edinburgh, Scotland, retired, USA citizen, country of last and intended future permanent residence - USA, intended stay in the UK - 3 months
Sowersby, Isabella, born 23 Dec. 1883, married, address in UK - 12 Redbraes Place, Edinburgh, Scotland, housewife, USA citizen, country of last and intended future permanent residence - USA, intended stay in the UK - 3 months