It's an internal TashTwenty board that keeps the floppy port on the back of the computer usable for daisychaining or a floppy drive. More details coming later today I hope...
Looks like a Mac Plus logic board: SCSI + 68000 + SIMMs but no ADB. SD interface occupying a ROM socket; so it's a SD-based SSD that interfaces on top of a ROM?
@robin-fo no third floppy port - it emulates an HD20 on an SD card (using Tashtari's TashTwenty) and then daisy chains the back floppy port out of the back of that - so it's like you have an HD20 and you're using the daisychain port on the back of the HD20. No need for a separate ROM (except on the 512k, which doesn't have HD20 support).