/usr/share/lyx/layouts/stdletter.inc is in lyx-common 2.1.4-2.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135  | # Letter textclass definition file.
# Author : Matthias Ettrich <ettrich@informatik.uni-tuebingen.de>
# Heavily modifed and enhanced by several developers.
Format 49
Columns                 1
Sides                   1
PageStyle               Empty
Style Standard
	Category              MainText
	Margin                Static
	LatexType             Paragraph
	LatexName             dummy
	ParIndent             MM
	ParSkip               0.4
	Align                 Block
	AlignPossible         Block, Left, Right, Center
	LabelType             No_Label
End
Style My_Address
	Category              "Postal Data"
	Margin                Right_Address_Box
	LatexType             Command
	LatexName             address
	ParSkip               0.4
	BottomSep             1.5
	ParSep                1.5
	Align                 Left
End
Style Send_To_Address
	Category              "Postal Data"
	Margin                Static
	LatexType             Command
	LatexName             letter
	KeepEmpty             1
	ParSkip               0.4
	BottomSep             1.5
	ParSep                1.5
	Align                 Left
	AlignPossible         Block, Left, Right, Center
	LabelType             No_Label
End
Style Opening
	Category              Letter
	Margin                Static
	LatexType             Command
	LatexName             opening
	KeepEmpty             1
	LeftMargin            Signature:xx
	LabelSep              xx
	TopSep                0.4
	BottomSep             0.4
	ParSep                0.4
	Align                 Block
	AlignPossible         Block, Left
	LabelType             Static
	LabelString           "Opening:"
	LabelFont
	  Series              Bold
	  Shape               Italic
	  Color               red
	EndFont
End
Style Signature
	Category              Closings
	Margin                Static
	LatexType             Command
	LatexName             signature
	LeftMargin            Signature:xx
	LabelSep              xx
	TopSep                0.4
	BottomSep             0.4
	ParSep                0.4
	Align                 Block
	AlignPossible         Block, Left
	LabelType             Static
	LabelString           "Signature:"
	LabelFont
	  Series              Bold
	  Shape               Italic
	  Color               red
	EndFont
End
Style Closing
	CopyStyle             Signature
	LatexName             closing
	LabelString           "Closing:"
End
Style cc
	CopyStyle             Signature
	LatexName             cc
	LabelString           "cc:"
	LabelFont
	  Series              Bold
	  Shape               Italic
	  Color               green
	EndFont
End
Style encl
	CopyStyle             cc
	LatexName             encl
	LabelString           "encl:"
	Spacing               Other 1.5
End
Style Location
	CopyStyle             cc
	LatexName             thispagestyle{firstpage}\location
	LabelString           "Location:"
End
Style Telephone
	CopyStyle             cc
	LatexName             thispagestyle{firstpage}\telephone
	LabelString           "Telephone:"
End
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