Perhaps this is the reason the II FDHD requires 6.0.2, due to the different pin configuration from the SIMM to the chip?

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Nope, did you know that the original release of System 6 released back in April 1988 was buggy? 6.0.1 fixed a lot of bugs and was originally what was supposed to ship with the IIx (and of course the II FDHD) which required it, but then another bug was discovered at the last-minute that led to 6.0.2.