I've just spent several hours checking between the various UK Census details, the Free BDM and Family Search as well as all your postings. John Powell is still single and living with his parents in Merthyr Tydfil in the 1861 census, so there is not much more I can find out about him before he immigrates in 1866, that will help link him indisputably with the John Powell you have found in the 1880 US census. You mentioned a 1870 record with John and Mary, what did it say?
I have found a Maria Aubrey working as a servant in London in 1861. There are lots of transcription errors on the typed page, but it can be seen quite clearly on the original. Such as Sarah Dewbrey, which is meant to be Aubrey, Shalonda Mannevy, which is meant to be Sophia Manning etc. Maria Aubury is listed as being a servant in the Relation column, when the original says "Niece" ... you get the picture!! However the original gives Maria Aubrey's birthplace as Berkshire .. when I want it to say Merthyr Tydfil, where I think she was bapt 30 June, 1837, daughter of John Aubrey and Elizabeth C057191. However I am also having trouble finding how she could be Sarah and Sophia's (whose maiden name was Aubrey) niece. Did they fill in her birth in Berkshire, because that's where she was born or because it followed on from what they filled in for themselves???
IGI has Sarah and Sophia both being baptised at St Mary Lambeth, Surrey, although Sophia has William and Mary as parents, whereas Sarah only has a mother named Elizabeth?

I can't link these women to John Aubrey, as I would need to do if they are claiming Maria as their niece. They are also closer to her grandfather's age then her father's.
Maria Aubrey's grandfather would seem to be the William Aubrey aged 50 watchmaker listed with the family John 30 Iron miner, Elizabeth 30 his wife and children Elizabeth 7, Maria 6 and David 2 all living around Merthyr Tydfil. So it's all a bit confusing!
My 94 year old Powell cousin is very intrigued with the US Powell women being seamstresses as some of the cousins in Australia were dressmakers too. However I think a lot of women at that time were dressmakers as we didn't have many factories making clothes like we do esp. in China nowadays.
Is there any way I can apply for a marriage certificate for John Powell and Mary? Would they be held by the Pennsylvanian authorities? That seems my only hope of making a definite connection across the Atlantic.

I should have a bit more time to work on this now things are settling down this end.
Also are there immigration records for people coming into USA or did these only start later with Ellis Island? I'm presuming you live in USA and so know more about these things?!?
Until next time, thanks for giving me so much of your time with my Powell family, it's much appreciated.
Dash
