/usr/share/augeas/lenses/dist/smbusers.aug is in augeas-lenses 1.2.0-0ubuntu1.3.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
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Module: SmbUsers
  Parses Samba username maps
Author: Raphael Pinson <raphink@gmail.com>
About: License
   This file is licenced under the LGPL v2+, like the rest of Augeas.
About: Lens Usage
   To be documented
About: Examples
   The <Test_SmbUsers> file contains various examples and tests.
*)
module SmbUsers =
autoload xfm
(* View: entry *)
let entry =
     let username = [ label "username" . store Rx.no_spaces ]
  in let usernames = Build.opt_list username Sep.space
  in Build.key_value_line Rx.word Sep.space_equal usernames
(* View: lns *)
let lns = (Util.empty | Util.comment | entry)*
(* Variable: filter *)
let filter = incl "/etc/samba/smbusers"
           . incl "/etc/samba/usermap.txt"
let xfm = transform lns filter
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