For some closure on this question from long ago, I stuck the SIMM into my SIMM programmer board. I changed the firmware slightly on my programmer board to leave the CS pin as an input because it's wired differently on this SIMM than on the other II-series ROM SIMMs. Then I dumped the contents of the chips...

It's exactly the same ROM that is on my 840av's soldered motherboard ROM -- checksum $5BF10FD1. So I didn't discover any exciting lost ROM versions or anything like that. :-( Oh well! Mystery solved.