By default, Drupal allows only users with the Administer nodes and view/reset/delete revision permission to add, reset and delete node revisions of all content type.
Node revision Permissions Per Role allows to give roles administer per content type node revision permissions without giving them full administer nodes permission.
For instance, you may let certain users(eg: roles:External-blogger ) manage node revision of a specific content type(eg: blog) but restrict to manage node revision of all other content type that this module is perfectly works for you.
Install the module like any other module.
Navigate to admin/people/permissions and set permissions for each role and each content type under Node revision Permissions Per Role.
If you do not see the permissions under permission page, please clear all cache of the site. After doing some modification of permission on permission page it is recommended to rebuild the permission.