On the IIci, I have recently confirmed with your experience that they are 8-bit and interleaved 1 byte at a time amongst 4 ROM chips. But a while ago when I interleaved the PEX FLASH ROM together, each flash chip was actually 32-bit, so I took 4 bytes from #1, 4 bytes from #2, 4 bytes from #1, 4 bytes from #2, etc. This may have nothing to do with the PEX ROM SIMM however.

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Are those the two PLCC chips on the board? Are they actually 32 bit wide chips? Or are they 8 bit chips and the board is reading four bytes at a time from each one. That's bizarre either way. In the first case, I didn't think there were any Flash chips wider than 16 bits around. In the latter case, I have trouble imagining the hardware to do that or why one wouldn't just use more flash chips to get the needed width.

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Yes, those are the 2 PLCC chips onboard. It is a strange setup. I'm not sure of the part numbers, but could they be serial instead of parallel? That would require feasible hardware to get it to 64 bits wide parallel data.

More clues though:
All dumps that I've made on PPCs are always different on different PPC Macs. None are identical like you might expect if this is some "emulation" ROM.

All dumps I've made contain the appropriate startup sound(s), including the TAM.

All dumps that I've made match checksums with those found at various places online.

There's no answer in what I mentioned, but maybe there are helpful clues.

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Every little bit helps.
Hmmm. The ROM dump on a 7500 should be the same as on an 8500, 9500, 7600, almost all the Mac clones (except the Tanzania based), and most of the 7200s. The ROM dump on an non-Kansas 8600 should match a non-Kansas 9600 and the 7300.

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Yes I'm sorry I forgot about that. In my ROM archive, I have several that have a whole bunch of names on them. It is reasonable that those series of Macs would actually have identical ROMs though.

Do you have a PEX then? I might have some more questions about it. Do you understand why there are onboard chips and a separate ROM DIMM? Was this some kind of early implementation of New World architecture?

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I don't have a PEX, but I was working with the ROM files that someone was extracting in a thread long ago. I believe that open firmware and the extra startup sounds were stored in the onboard chips. The normal startup sound and the rest of the ROM was stored in the DIMM.

I haven't really been keeping track of the PEX discussion. I got into this because the other fellow found my name in relation to Beige ROM modules and ocntacted me. And I said, "Sure I can build you a PEX module." But it's not working out as easily as I expected. For one thing, I can't get the sockets to work. I've built about 200 of these modules which worked, so it should be easy. But those were all with soldered down chips. With the sockets, I haven't had any luck yet.
So I'm falling back to building a module with soldered down chips, but that makes it extra important to resolve any issues with the correctness of the code.

Sigh. To make matters worse, I've been working on this (off and on, mostly off) for almost a year. I told him I was busy and would be slow, but this is ridiculous. I really want to get this done. But I also want it to be right...

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Do you have an original PEX ROM DIMM? If not, maybe you could find the guy from that old thread and get him to take a high-resolution picture for you. That could help spot any differences.