Well guys, it turns out that this IS one of those "Oh, yes, on that revision board you need to do X" things!
Armed with some schematics, a multimeter, some tape and a pen, I traced out the decoding logic.
Everyone knows that to upgrade from 6 chip to 2 chip, you desolder the 256k pads, bridge the 1M pads, pop a 7411 in U68 if needed and plonk your ROMs in to sockets U67 (LO-0) and U63 (HI-0) right?
Nope! Not on a C070789-001 REV B you don't. You see, the output of the 7411 (the 1M CE line) doesn't go to pin 20 of those sockets to enable the ROMs.
It instead goes to the HI-2 (U48) and LO-2 (U53) sockets! Exxos notes in The LaST Upgrade Part 1 that this can be achieved on REV C boards by doing a mod. No mod has been performed on this board. That's just the way it's factory wired!
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ROMs installed in the correct place for this board revision
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Looking good!
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There's that Rainbow I was chasing
She's only got 512K of RAM so I think an upgrade might be next