The ROM SIMM slot is for upgrading Macintosh IIsi ROMs. There are
actually two versions of the logic board being shipped as of December
1990: One type has the ROM surface-mounted to the logic board; the other
does not have this ROM at all (only a space for it) but does have the SIMM
installed.
The rule-of-thumb for the ROM SIMM: If a board has it, don't remove it;
if it doesn't, don't put one in.