There is no doubt that Ann's patrilineal line great-grandfather, David DAVIES (1855-1925), was a Welsh coal miner — living in the village of Nantyffylon, near Maesteg, Glamorgan, Wales, as shown in his census records.  The issue is whether his son, David Thomas DAVIES (1882-1955) — the immigrant in 1927 — was actually an impoverished coal miner.

One such assertion is made in an article at BBC News:  Ann Romney's Welsh Ancestry Explored, where is says,

The David Thomas DAVIES living on Victoria Street in the 1911 census of Caerau, Wales, is not the paternal grandfather of Ann (DAVIES) ROMNEY.  Her grandfather appears to be missing from the 1911 census, so we do not have a record stating he was a coal miner.  We may get one when the 1921 census of Wales is released in 2022.

In the ship's manifest of 1927, when Ann's grandfather, David Thomas DAVIES, made the crossing from Southampton to New York City, his occupation is given as "mechanic."  In 1928, he filed a Declaration of Intention in the U.S. District Court in Detroit, MI, where he indicates his occupation is "mechanic."  In the 1930 U.S. Census, the family was living in Detroit, MI, and his occupation is given as pipe fitter in an auto parts factory.  In 1934, he filed a Petition for Citizenship in the U.S. District Court in Detroit, MI, where he, again, indicates his occupation is mechanic.  In the 1940 U.S. Census, he has no occupation, but his eldest son is living with him and gives his occupation as engineer in auto manufacturing.  His son, of course, went on to achieve considerable financial success.

(Note:  also in the 1930 U.S. Census, our subject indicates he served in World War I — obviously for Wales.  It seems likely to me that our subject learned the skills of mechanic while serving in the military.)

Both of our subject's parents, David DAVIES (æ 57, b. Maesteg) & Elizabeth DAVIES (æ 56, b. Nantyfflon), are living.  They are enumerated in Nantyffylon, with their children, William Ed DAVIES (æ 27, b. Nantyffylon) and Emris DAVIES (æ 21, b. Nantyffylon).  Our subject was also born in Nantyffylon and this definitely is his known family; unfortunately David Thomas isn't with them.  We should not be surprised that he isn't because it's not unusual for an adult unmarried male (æ 28) to be "working out," as it's called in genealogy.  If he's moving from job to job or boarding somewhere, it's easy to get missed in the census.

Our one chance to find him giving occupation, "coal miner," may come when the 1921 Census of Wales is released in 2022.

There are over 7000 David DAVIES'es indexed in Wales in 1911, with over 2500 of them born in Glamorgan.  As David Thomas was born in Nantyffylon, Maesteg, Glamorgan, in 1882, I confined my search to David's born in Glamorgan within two years of 1882.  If his birthplace is entered incorrectly on the census forms, I don't believe there's any hope of finding him — or proving it's him if we do find an otherwise plausible record.

I did one of the most exhaustive census searches (in the index at Ancestry.com) I have ever done — up to and including browsing through every page in the Enumeration District where his parents were living (Cwmdu, ED 7).

The index at Ancestry is very hard to use with regard to locations, so to be absolutely certain I hadn't missed some David Thomas DAVIES mentioned in the BBC New article, I browsed the census records for the full length of Victoria St., from the REES family at #1 (Image 182 of 831) to the MORGAN family at #102 (Image 411 of 831), and found no reasonable alternative.  So, the BBC News article was, indeed, referring to the one at #13 Victoria Street. 

I've included the results of my search in a table, below, as confirmation that I did the search and in case it might aid others working on DAVIES in some way.

In the 1911 census, a location could consist of a Registration District and a Registration Sub-district, each with a name and number, and a numbered Enumeration District.  It would have had both a Civil Parish and an Ecclesiastical Parish. It could be Ward within a Borough or a Borough within an Urban District, or it could be in a Rural District.  It would be in a parliamentary borough or division, and it would have a postal address.

It doesn't help that birthplaces in the census don't seem to be consistently given.  It appears that when asked their birthplace, the same individual, from year to year, may give the village or parish or district of the county in which they were born, with no indication which it is.  It makes using the search engine at Ancestry.com to find someone by their birthplace very difficult.

As a further complication, one name (e.g., Maesteg) might refer to the village or borough, the urban district or borough, the registration district or sub-district, or civil parish.  The fact that most genealogists drop the qualifiers on locations — a very bad practice promulgated first by the LDS and, now, by Ancestry.com — means locations given in genealogies are all too often ambiguous.  For genealogical purposes regarding Wales locations, it would be most useful if the focus was on the village name — so stated as the village name — as the most precise and generally unchanging name of someone's location.