I have an 040 accelerator kit that includes a ROM SIMM that fits in that slot. I don't think Apple ever actually did anything with it, though. I only bought the kit because it included cooling fans for both the 68040 and a 601 upgrade board if one was present, which I have. I also moved my Q700 into a IIvx case so the hard drive shelf doesn't allow the ROM SIMM to be installed because it stands up too high. I don't need the 040 accelerator, anyway, just the cooling fans.