If you mean the chip socket on the board that mates with the logic board, it is a FPU socket. That ethernet card worked in both the SE/30 and the IIsi. The SE/30 has an FPU soldered to the logic board, so the socket is useless in an SE/30. However, the IIsi lacked the FPU and so the socket was a handy way to add an FPU. It seems like just about every card that could ever plug into the IIsi PDS slot comes with a socket for an FPU.
There's always SimpleText or maybe back then it was called TeachText. Simple as can be, but ought to fit in the available space. Maybe an older version of MacWrite would fit?
There was a utility called RAMDisk+. I don't know if it worked on 7.5 though. I definitely used it on all versions of 7.1.
Jeff Walther