Here's a link to a site that has USGI Ordnance Field manuals.
The Carbine manuals show full disassembly of the carbine including the trigger group.
Since the Iver Johnson used the GI design, the instructions work for it too.
NOTE the infomation near the top of the page on what user name and password to use to get entrry:
http://www.biggerhammer.net/manuals/