This is the wooden forearm assembly. You can see the accelerator spring assembly that is held in place by the sling swivel stud. If your friend is building his carbine with a coil spring mainspring (rather than the old model leaf spring), this accelerator does not need to be added to the forearm. A stiffer mainspring will do the job. The 1920 Carbines eliminated this feature.
http://boards.rennlist.com/upload/Forearm.jpg