Say what?

Okay, so I've completely disassembled my recently acquired Quadra 950 for a thorough cleaning and I'm wondering about something...

On the Quadra 950, there are 16 30-pin SIMM slots and there are 4 VRAM SIMM slots. So what's that empty SIMM slot just behind the 16 RAM SIMM slots for? It looks exactly like a VRAM SIMM slot it's not. The 4 VRAM slots are off to the side. I've been Googling to the point of total Google exhaustion, I give up... What's that slot for? A ROM SIMM?
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Wrong answer? No, I gave the right answer. You must have asked the wrong question.