It's a pin that runs along the top of the slide.
I'm working from memory, but I think it's connected to the firing pin or striker in some way, a two- or three-part affair, protruding from the rear of the slide when the chamber is loaded.
I contributed to some of those older threads, as I had a relative's Behorden for a while, and it was missing part of the loaded-chamber indicator. I believe that someone posted a schematic with the parts called-out by number.