Well the thread_view template is the main template and if you open it in the ACP you will see all the other templates it pulls from - there should several rows of tabs, the message template being one of them.
Which in turn includes the message_user_info template, which is the one you need
If you use Firefox and Firebug you can use that to inspect a specific element on the page and again just look for the class which will denote the template.