Her birth index record uses exactly the same names as Arm Fam, so has to be her:

Surname First name(s) District Vol Page
Births Mar 1870   (>99%)
Clarke Marion Sarah N Northwich 8a 231

And the precise date is from the 1939 register.

Her death index was found (in 2019) on FreeBMD:

Surname First name(s) Age District Vol Page
Deaths Jun 1955   (>99%)
BUSFEILD Marion S N 85 Radnor E. 8c 396

and from this also on FindMyPast:

First name(s) MARION S N
Last name BUSFEILD
Birth year 1870
Age 85
Death quarter 2
Death year 1955
District Radnorshire East
County Radnorshire
Volume 8C
Page 396
Country Wales
Record set England & Wales Deaths 1837-2007
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She was found in the 1911 Irish census, with her mother and her sister and in the home of the latter but not with her husband.  This was in Maidenhall Danesfort, Kilkenny.

Surname Forename Age Sex Relation to head  Religion  Birth place
Butler George 51 Male Head of Family  Irish Church  Kilkenny
Butler Harriet Neville 43 Female Wife Church of Ireland    England
Butler Cicely Marion 12 Female Daughter Church of Ireland    Kilkenny
Butler Joanna Vernon 8 Female Daughter Church of Ireland    Kilkenny
Butler George Gilbert 1 Male Son Church of Ireland    Kilkenny
Clarke Mary Elizabeth 74 Female Mother in Law  Church of Ireland  England
Busfeild Marion Sarah Neville 41 Female Sister in Law  Church of Ireland  England
Murray Frances Elizabeth 61 Female Visitor Church of England        England
Rowe Sarah Jane 18 Female Servant Roman Catholic           Waterford
Mollison Jane Ann 50 Female Servant Free Church of Scotland  Scotland
Egan Ellen 27 Female Servant Roman Catholic           Limerick
Burke Julia 31 Female Servant Roman Catholic           Tipperary
Kelly Mary 23 Female Servant Roman Catholic           Kilkenny

This gives her mother's married name as Mary Elizabeth Clarke; hopefully this may lead to her marriage and to her parents.
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A close relation of hers was the mother of Antony F Mount whose birth index was:

First name(s) ANTONY F
Last name MOUNT
Birth year 1950
Birth quarter 2
Registration month -
Mother's maiden name Clarke
District Crickhowell
County Breconshire
Country Wales
Volume 8A
Page 59
Record set England & Wales Births 1837-2006

His parents' marriage was probably one of two Mount-Clarke marriages:

Mount Christopher J — — 1947   Hay, Breconshire
Mount Dennis — — 1952  Northumberland Central, Northumberland

My guess is that it was the first:

First name(s) CHRISTOPHER J
Last name MOUNT
Marriage quarter 1
Marriage year 1947
MarriageFinder CHRISTOPHER J MOUNT married one of these people
Audrey M Clarke
Spouse's last name Clarke
District Hay
District number -
County Breconshire
Country Wales
Volume 8A
Page number 103
Record set England & Wales Marriages 1837-2005

Cristopher J Mount appears as:

A Flight Lt Christopher John Mount was awarded the DFC for conspicuous gallantry in 1940.

He has no other events on FindMyPast, no birth and no death.

Audrey M Clarke, on the other hand, has over 300 events listed on FindMyPast for people of that name born anywhere from 1910 to 1920 (the last of whoich would have made her 17 at marriage).

In the 1939 Register there was this Audrey M Clarke living in Hay, Breconshire:

Audrey M Clarke's life in 1939
Treble Hill, Hay R.D., Breconshire, Wales
Clarke household (8 people)

First name(s) Last name(s) DOB Sex Occupation Marital status  Schedule  Schedule Sub Number

Marshal F Clarke 03 Feb 1876 Male Lient Col (Late Chesire Regt) (Retired)  Married  53  1
Olive S Clarke 19 Dec 1885 Female Domestic Duties  Married  53  2
Audrey M Clarke 04 Sep 1913 Female Temporaly Incapacitated After illness   Single  53  3
Amy E Garnett 29 Sep 1861 Female Householder Here Temporaly  Widowed  53  4
Gwyneth R Phillips (Williams) ? Jun 1922 Female  House Parlourmaid  Single  53  5
Peter R Garrett  07 Sep 1930  Male  At School  Single  53  7

If this was her, she would have been 34 at marriage and 37 at the birth of her son Antony.

In 1948 there was another Mount born to a Clarke mother in Crickhowell district:

Last name First name(s) Year Of Birth Year Of Death  Year  Record set  Location

Mount David R 1948 — 1948 England & Wales Births 1837-2006  Crickhowell, Breconshire, Wales
Mount Antony F 1950 — 1950 England & Wales Births 1837-2006  Crickhowell, Breconshire, Wales

On the 2002-2018 Electoral Rolls, they were both in Ascot:

First name(s) Antony F
Last name Mount
Year 2010- 18
Street Bagshot Road
Town Ascot
County Berkshire
Country England
Postcode SL5 9JL
Age guide 65+
Address The Garden House, Bagshot Road, Ascot, Berkshire, SL5 9JL
District Ascot
Occupancy 8  (years)
Other occupants David Mount
Electoral rolls 2010, 2011, 2012, 2013, 2014, 2015, 2016, 2017, 2018
Company director -
Record set Uk Electoral Registers & Companies House Directors 2002-2018

And in his own electoral roll entry David has the second initial of R.

This confirms to some extent that I have found the right parents.
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Her probate index - searched because her death was not to be found on the GRO indices:

BUSFIELD Marion Sarah Neville of Boultibrooke Presteign Radnorshire (wife of Johnson Atkinson Busield) died 7 May 1955  Probate Birmingham 4 July to the said Johnson Atkinson Busfield retired major H.M. Army.  Effects £6046 18s. 11d.
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Her death index could not be found originally but the above led to the entry curiosly under the surname of Busfeild, not previously associated with her husband.
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Their marriage index:

First name(s) Johnson A
Last name Busfield
Registration year 1907
Registration quarter Jul - Sep 1907
Registration district Celbridge
Volume 2
Page 419

MarriageFinder ™
Johnson A Busfield married one of these people
M S Neville-Clarke, Marion Sarah Neville Clarke
County Dublin, Kildare, Meath
Country Ireland
Record set Irish Marriages 1845-1958
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