Yeah, its for a ROM SIMM. All LC475's have those solder pads for it, but I've never seen an LC475 with the actual slot, I'd say that as Gil said, it may have only been in prototype or very early production units. The LC475 came out in October 1993. My LC475 was built that month, and doesn't have one, it just has the pads like the machine on that page, so I'd say that machines with that slot would be fairly rare.