These are knurled steel track pins. They should have a 'head' but they don't, but this is the concept that prevents track pins from working their way out of the link. It's worked for years, starting with Impact and now DKLM, which is photoed. This system does not work well with 'closed end' track links, but with the dust, dirt and mud no one will notice whether the track link is 'open' or 'closed'. Pins should be inserted from the outside of the link whether they are knurled or have heads, reason being that if a pin starts to work it's way out it can be seen and fixed. Inserted from the inside, tank hull side, the driver usually does not notice the pin until it begins to catch on the hull.