Hi Mike,
The 1861 census was taken on 7th April so it's just possible that your Joe could have married the 3rd Miss Harris although like you, I think it's unlikely! The second one looks more plausable, not only because of the date but also the middle name (Julia M HARPER in the census).
Portsea is on the South coast of England, roughly in the middle and near the city of Portsmouth so some distance from both Kent and Gloucestershire. There again, he was a Railway Guard in 1871 so he would have traveled about a bit.....
It just occurred to me to have a look at the 1881 census for Joseph & Julia and...
1881 Census, Portsea, Hampshire (RG11/1158 Folio 39 Page 28)
Juia HARPER, Boarder, Widow, 56, Annuitant, b. Devon, Sidmouth
The head of household is one Richard TING, Property House Owner.
I think Railway Guards used to stand in the last carriage of a train (where freight was carried) and blew a whistle as the train moved off.
Regards,
Al, In Mexico