I think the 10 Aug 1870 census is the only thing we can hang our hat on regarding Arthur Mould. At 3 months old, he was born around May 1870, conceived around Aug 1869. So I think the timeline is:
- Aug 1869: Arthur is conceived by Thomas Mould and Jane Spencer
- ~Sep 1869: Thomas and Jane part company (she's not a widow, because we have a death certificate for Thomas who died in 1874 in England). Jane and daughter Clara sail for New York, listing their Surnames as Spencer.
- 7 Oct 1869: The SS Manhattan arrives in New York
- May 1870: Arthur is born in Utah as Arthur Spencer
- 21 Aug 1871: Jane marries William Ashdown
- 1880 census: Jane and William Ashdown are still in Utah. Clara's name is now Clara E Ashdown. Arthur (who would be 10) is not in the household, presumably dead as Arthur Ashdown.