This is just the bolt that you push on to advance and retract the mechanism for the Bolt Actions, Slim Bolt Actions, and the Pencils. Please be sure to select the correct model as the Standard and Slim versions differ in size.
Note: These parts are threaded, so unscrewing the old one and screwing the new one in tightly is required. We use Knipex smooth jaw pliers to install these so they won't come out very easily. Be careful if you are removing your bolt with hardened steel pliers as it is easy to scratch the pens.