/usr/share/gconf/schemas/apturl.schemas is in apturl 0.5.1ubuntu3.
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 | <?xml version="1.0"?>
<gconfschemafile>
  <schemalist>
    <schema>
      <key>/schemas/desktop/gnome/url-handlers/apt/enabled</key>
      <applyto>/desktop/gnome/url-handlers/apt/enabled</applyto>
      <owner>apturl</owner>
      <type>bool</type>
      <default>true</default>
      <locale name="C">
        <short>Whether the specified command should handle "apt" URLs</short>
        <long>True if the command specified in the "command" key should handle "apt" URLs.</long>
      </locale>
    </schema>
    <schema>
      <key>/schemas/desktop/gnome/url-handlers/apt/command</key>
      <applyto>/desktop/gnome/url-handlers/apt/command</applyto>
      <owner>apturl</owner>
      <type>string</type>
      <default>apturl "%s"</default>
      <locale name="C">
        <short>The handler for "apt" URLs</short>
        <long>The command used to handle "apt" URLs, if enabled.</long>
      </locale>
    </schema>
    <schema>
      <key>/schemas/desktop/gnome/url-handlers/apt/needs_terminal</key>
      <applyto>/desktop/gnome/url-handlers/apt/needs_terminal</applyto>
      <owner>apturl</owner>
      <type>bool</type>
      <default>false</default>
      <locale name="C">
        <short>Run the command in a terminal</short>
        <long>True if the command used to handle this type of URL should be run in a terminal.</long>
      </locale>
    </schema>
  </schemalist>
</gconfschemafile>
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