So they used the exact same solder pads, but turned it into a RAM SIMM slot instead?

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The pinouts are very likely not the same, but they're also very likely not tremendously dissimilar. They're definitely not 16 bit - not sure what you were referring to, Trash80toHP_Mini, but if you're talking about address lines, the ROM slot would at very minimum support up to 4 megs, which would require 20 address pins (the ROMs are 32 bits wide, so 2^20 would give 1 million 32 bit words, or 4 megs).

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I was going from my eratically behaving memory. I guess I assumed that the RAM SIMM was 32 bits wide, not 72, and the next steps down would then have been 24 or 16 bits, as I stated . . . incorrectly.

The re-arrangement of the dissimilar pinouts was the reason for choosing a wire wrap socket for the hack. You'd cut to nubbins anything that wasn't identical and solder rework wiring to nubbins for the dissimilar/non-existent lines.

The problem with adding more slots is that you need to generate chip select lines based on the address space being accessed. It's possible that the massively integrated chips on the Quadra 630 type machines could support more than 256 megs, but if nobody has documentation about them and their pinout, it might be difficult or impossible to ever do this.

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Exactly, but the 68040 should have plenty of memory select lines available and it's very well documented. The Pro Level Quadras have at least four RAM slots, do they not? It wouldn't be as easy as hacking a IIfx by hijacking the existing parity lines with a few cuts and very short rework wires, but it might be possible. You'd need to be careful about keeping all the lines at similar lengths to avoid timing issues . . .

. . . but it might be a lot of fun to look into! }

My IIfx memory hack notions take precedence though. :rambo: