I have had the same suspicions about Donovan/Helm since I heard there was a marriage in SA. Eliza still has grandchildren living, however, and they have not mentioned this to me; although this does not mean it is not so.
Helm could easily be Holm, I guess, or could have been transcribed mispelt as Holm, Holmes, Hellem, etc. The only thing is, on every single birth index and record her name is given as the same, Eliza Helm. Even when John is 'Owen', she is Helm. She is the signatory on birth entries so gave her own info (obv she signs as Owens). Also, her name was known in the family as Helm, even before we started shaking the family tree.
Can I post a copy of her signature up here? Is that allowed
