I know we already talked on PM and you've already seen my post, but just wanted to post this here for future reference.

I happened upon the ROM SIMM you're looking for on eBay and posted about it here.

That socket you have circled is indeed the ROM SIMM socket, and I believe the unpopulated pads for the two chips below it are where the motherboard ROM would have gone. I'm just going to save some of the determined pinouts and sizes/shapes from the SIMM for future reference and then I'd be thrilled to send it to a good home where it will be used!