/usr/share/texmf-texlive/makeindex/mkind-english/digit.ist is in texlive-extra-utils 2009-10ubuntu1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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%
% This file is part of
% MakeIndex - A formatter and format independent index processor
%
% Author: Gabor Herr
%
%
% $Log: digit.ist,v $
% Revision 1.1 1991/12/13 16:43:46 schrod
% Initial revision
%
% Revision 1.1 1991/08/26 17:50:00 herr
% Initial revision
%
%
% english sort rules for digits
% sort order is ASCII, digits build a group
sort_group "0" 100
sort_group "1" 100
sort_group "2" 100
sort_group "3" 100
sort_group "4" 100
sort_group "5" 100
sort_group "6" 100
sort_group "7" 100
sort_group "8" 100
sort_group "9" 100
% define group headings
group_head 100 "Numbers"
group_head -100 "numbers"
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