Brian, what metal part? Are you talking about the prong piece that is attached by two screws at the back of the lens.
If so look and see if the two screw holes are still there. If so then look for the cutouts in the material around the lens mount. It could be that someone removed the prong.
There are also two types of manual lenses that were made without the prong. The first type would be Series E lenses. They were made after the AI and AIS lenses were made. They are fine. Nikon also made late model AIS 50mm f1.8 lenses without the prong.
The second type are preset lenses. Since the aperture is set manually the lens then meters in stop down mode which does not use the prong. Generally these lenses are fine except check the manual Incompatible Accessories and Non-CPU lenses list which lists certain lenses by serial number that are not compatible.