I think that there is some confusion here....Note the four socketed DIPs next to the SIMM sockets. We can assume that the IIx layout is similar, because the ROMs were sold as an official upgrade to Mac II owners. ...Repackaging the SIMM ROM types in DIPs is no doubt achievable but you need a lot of motivation to try it.

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Not really ... I am more interested in the encoded software compatibility than the physical compatibility. Having said that, the IIx ROM was a SIMM. While it makes sense to us that the IIx should have used the same layout as the II, especially considering their plans to allow for an upgrade, they essentially did the same thing with the SE/30 & SE. For whatever reason they got a wild hair that the ROM's would work better as SIMMs than DIPs. Why on Earth they would have soldered the ROMs in place on any machine is beyond me – lord knows by that time they had gotten enough ROMs wrong the first time around ... perhaps that was a way to limit the Mac from being upgraded. The Portable had the configuration as well.