Thank you, yes, I know about the other thread. I have been working with that poster (another relative) on this for years. I deliberately kept the information brief as I was hoping for opinion/experience re why and when surnames are used as middle names.
Personally I don't think the neighbour is significant (at least we have not been able to find anything to link him to the Charles Joseph Eldridge Jones). We have done loads of work on the theory that he is illegitimate and that his name offers a clue to his father without success. Anyway would he really want to preserve evidence of his illegitimacy by giving the name to every one of his children? I tend to think that there is another explanation, maybe social climbing, perhaps Eldridge Jones is more aspirational than plain old Jones (my last name is Jones and I wouldn't swap it for anything!). It does seem that it was thought of and used as a double surname. Or it could be a way of preserving the maiden name of a mother or grandmother, I don't know.