Her only surviving brother, Charles Shaw-Lefevre, barrister, had an eldest son Charles who became Speaker of the Commons and was made a viscount on resigning as an MP in 1857.  The viscount had no surviving sons and the viscountcy died with him.  But his grandson George John Shaw-Lefevre was also an MP and was made a baron in 1906, but died with no surviving sons so that barony is also extinct.
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Her burial: (Birmingham & Midland Soc for Gen & Her):

First name(s):             Maria
Last name:                 POWYS
Date of burial:            31 Dec 1833
Age at death:              82
Calculated year of birth:  1751
Place of burial:           Leamington Priors
Dedication:                All Saints
County:                    Warwickshire
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Her baptism has yet to be found and her birth date is unknown.  Her father had been appointed curate at Falkirk in 1748, so she might have been born there.  Her father’s next appointment on the CCEd was as rector of Kirk Smeaton in 1759.
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And this indeed led to a Result:

“            A Copy of the Register at Felkirk AD 1751
“                           Xtenings
“April
“. . . .
“  15 Mary Daughter of ye Revd. G. Shaw & Mary his wife  by J Hicadley”

Note that this adds in the forename of Mary (Maria’s) mother.
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The transcription by FindMyPast is:

First name(s)           Mary
Last name               Shaw
Birth year              -
Baptism year            1751
Baptism date            15 Apr 1751
Father's first name(s)  Revd G
Mother's first name(s)  Mary
Baptism place           Felkirk
County                  Yorkshire (West Riding)
Country                 England
Archive                 Borthwick Institute for Archives
Record type             Bishop's transcripts
Register type           Baptisms, marriages & burials
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Frome the Northants FHS bookllet of Mon Inscriptions for St John the Baptist, Thorpe Achurch, pub 2015:

ON a marble memorial:
“Sacred to the memory of Maria, widow of the Reverend LITTLETON POWYS, formerly Rector of this Parish.  She died December 21st 1833 aged 82, having been enabled to fulfil with exemplary fidelity the various duties which the providence of God had called her.  She exchanged this mortal for an immortal state having a good hope through grace and trusting in the above merits of Christ the Saviour for pardon and acceptance at the eternal throne.  This tablet was erected by her grateful and affectionate children to the best of mothers. “Blessed are the dead which die in the Lord.”  “Even so saith the spirit for they rest from their labours and theri works follow them.”
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The transcript of the image of the parish register page, by me:

[                Westminster > St Marylebone > 1777-1782

“                ( The Year 1778 )              Page 153
“No 4576 The Revd. Littleton Powys  of the parish of Achurch in the County of Northampton Batchelor and Maria Shaw of this parish Spinster were Married in this Church by Licence
this thirteenth Day of October in the Year On Thousand Seven Hundred and Seventy Eight by me  John Temple
  This marriage was Solemnized between us  Littleton Powys  Maria Shaw
  In the Presence of  Geo Shaw  Ann Glover
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On of the indexes to this page of the marriage register gave the full address of the church as:
Saint Mary,
St Marylebone Road,
St Marylebone,
London
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