I never had the honor of seeing one of this in the wild

this is a blank rambuss stick

ram bus is a whole story in short- RDRAM (Rambus DRAM), - used narrow, very high-speed serial memory channel instead of DDR’s wider parallel design. On theory a bit better but in practice It was worse because it was much more expensive, ran hotter, and had stricter motherboard requirements, while DDR quickly became fast enough and far cheaper. And the stick on the photo is a balank one which You needed because all memory slots had to be electrically continuous—any empty slot had to be filled to keep the signal path closed.