Macintosh performa 475 simm slots

[Mike's Future Retro]

I recently acquired a performs 475. Can some one answer what goes in the empty simm slot ? The simm slot on the top row is regular 80ns ram, then a slightly smaller width simm slot (empty in the picture) then 2 simm slots for video ram… what’s that empty slot for ???

 

[dougg3]

It's for a ROM SIMM that could override the soldered-on ROM chips. Most 475 boards I've seen don't have that socket populated; you might have an early board.