There was a Mr W.A.Walker departed London on 18 April 1928, going to Beira, Mozambique. He was aged 17, a farmer from Castle Craigs, Inverurie. The ship was the GLOUCESTER CASTLE.
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On 5 Jun 1931, he appears to have made a return journey. William Walker, aged 20, a Storeman, arrived Southampton from Lourenco-Marques, Mozambique. He was on the WANGONI, and it says he was going to Cuttle Craigs, Aberdeen. I am guessing that they mean Castle Craigs,
as on the voyage above.
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An Alex Walker, born abt 1891, was aged 43, arriving 25 Oct 1934 at Southampton from Lourenco Marques, Mozambique, and I wondered if this was his father. His wife was with him...Anne Walker aged 42, and daughter Anita, aged 11. They were on the WATUSI, going to Kirkeville, Kirn, Argyllshire, and it says they had been in N Rhodesia.

I am not sure if any of these are the correct person...what do you think yourself?
Brian