That there is a ROM SIMM slot. I too, also having a Quadra 700, have noticed this. they were originally put onto the Quadra's for a ROM upgrade, but as far as I know there are none.As for the unknown SIMM and slot -- here's a diagram of the motherboard:
From the Quadra 700 Developer notes:
"The MCU controls the Macintosh Quadra 700 1 MB ROM, whichconsists of two 4-Mbit ROMs (each ROM is a 256K x 16-bit, 150-nsdevice). A ROM SIMM socket is available for future expansion;however, when a ROM SIMM is installed in this socket, the on-board ROM will be automatically disabled.
looks to me as if it was a possible factor to them, but they were never made. I do wonder if it's possible to put a IIci or some other type of ROM SIMM in there; that might be a possible up(or down)grade.