Thumbing through a copy of "Guns of the Elite" by George Markham, noticed several photos of M16s with rather goofy looking upper receivers, seems to be a different type of forward assist, short knob projecting out of the rear most portion of the upper receiver, about 1.5-2" long, with what appears to be a forward assist button. Page 146 and 111 if you have the book, any ideas?