/usr/share/lyx/layouts/extbook.layout is in lyx-common 2.0.6-1build1.
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 | #% Do not delete the line below; configure depends on this
# \DeclareLaTeXClass[extbook]{book (more font sizes)}
# Extended book textclass definition file.
# Herbert Voss <voss@lyx.org>
Format 35
Input book.layout
# change available fontsizes
ClassOptions
FontSize 8|9|10|11|12|14|17|20
end
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