this later revision of the st-251 uses a mask-ROM microcontroller. it's very similar to the R6518, at least in the pinout. it also latches its internal address bus on a bunch of the pins during the rising edge of phi2! this means i can spy on the program counter and any other memory addresses being accessed.
the firmware from the older board revision (which uses an external EPROM chip) has been dumped and (mostly) analyzed! see https://github.com/schlae/HardDriveInfo/blob/main/st251/firmware/ST251_commented.txt