It's a Kart bushing installed by Chambers Custom.
I sent it to Chambers for a fit/function check and he tuned the ejector and installed the Kart bushing to ensure that he could fit the barrel lugs to his satisfaction.
That gun passes The Barrel Fit Challenge with flying colors!
ETA: Forgot to mention that I'm the original owner of that gun. It was carried some and shot not much. It always ran 100% reliable but I wasn't totally satisfied with the initial wear pattern on the slide stop, so instead of trying to get Wilson to fix it, I killed it with fire and sent her to Chambers. He blueprinted the whole thing and said it was damn nice, just needed a minor tweak to the ejector nose and a bushing to bring the barrel fit up to his satisfaction.