Wraith and Lancer, the brown piece on the MP38/40 is a hand guard made out of a form of bakelite and can be found in colors that will be either brown or black. The guard actually reaches around and underneath the weapon from the magazine to the pistol grip. It is put there to provide a insulated method to hold the weapon where it is less likely to cause jams when firing. The prevented jams usually occurred by holding the magazine or by grabbing the barrel which is also painful (personal experience). It also helps with field stripping the weapon to clear jams, cleaning, etc.
Hope this helps.