Just to explain what you are seeing there - the Code content type been deprecated as of Matrix 5.1, which means it's liable to be removed in any future minor release (so 5.4.0.0, 5.5.0.0 and so on) as per our deprecation policy. That's why it's not on the Manuals page, as generally speaking we don't allow creation of new deprecated assets in our interfaces.
It continues to live inside Matrix until a suitable automated and lossless upgrade path is worked out.
If you're still using the Code content type in a pre-5.1 Matrix system, you can still change the version number of the manuals to "5.0" (or earlier) to get the applicable manual for that version. As you can see, it's been blanked out in the deprecated versions.
Hope this helps, even if it's not the answer you were probably looking for.