Not sure if this helps, but there is a marriage record for Duncan Elmhirst Mackellar on Find My Past.
On 14 May 1846 at Calcutta Fort William he married Harriet Emma Cullen Martindell, widow, daughter of Charles Henry Penrose. Groom's father is Duncan Mackellar. Groom was a bachelor and mariner. Abode of both was Calcutta. Witnesses A. Rogers, Geo maxon, E Hailes, E L Maxon