Published in the Horry Herald:
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Allen, Gaston to Johnson, Hattie, 25 Jan 1903 at residence of B.J. Graham near Blanche by Rev. A.B. Rowe
Norris, D.W. to Allen, Hattie, 14 Oct 1911 by W.L. Alford
So Hattie didn't marry Duke until more than a year after Sandy was born - I would guess that's why he went by Allen.
Wonder who the father of eldest son Doctor "Dock" Henderson was??