Yes, the extra chip is a X 1 chip for parity. If you scavenge from a 128MB SIMM you get a bunch of 16M X 1 chips as a by-product. That's why I built both two-chip and eight-chip IIfx SIMMs. The X4s are a lot easier to come by, but they require me adding a couple of ICs to the SIMM to handle the IIfx's special write buffering scheme. That's not an issue on any other Mac, so one can just use the X 4 chips by themselves.