I said that up top. Recalled because of a socket issue. The four chips to the lower right, see those? The black upper right one comes in versions that have different pin layouts underneath - the HMMU (Memory Management Unit). Important if upgrading to a PMMU which would not fit. When Apple made the first short run, the socket was specific to the chip they used so upgrading was impossible without soldering in a new socket. Without a PMMU, the Mac II can not use 4MB SIMMs.

It gets even more complex since there are two sets of SIMM banks and they are not the same. If you dont upgrade the ROMs for Superdrive support, bank A can not even use high-density SIMMs. Its really an odd Mac. I got around most of that by just putting in an 030 Daystar upgrade.