My FDE model has what looks like a nut as in a threaded nut. Never noticed this though. Interesting to find this now. I've owned mine for close to 6 months and cleaned several times. I'd say around 5 times. Guns, especially modern guns are purpose built machines that can run effectively even under soiled conditions. Obviously a quick swipe of the bbl with a snake or the like is preferred, when conditions allow. Guns were made for war of some sort, I'm not claiming to have been in combat, but to design a gun that MUST be cleaned after each use is a bad design. However, I have a thing about keeping my tools, the expensive ones anyway, clean as I can. Plus I enjoy stripping the gun down and cleaning it up. It's therapeutic of sorts I suppose. Having said that though some do believe that the average gun owner/cleaner can do more harm then good trying to clean one. Thanks for starting this though I may have never noticed that, really hope there is a solid answer to what all the different stamps mean.