another fun one is the empty space for an EPROM and a latch chip. these weren't used to store drive firmware (as i thought before) but they store a giant lookup table of microstep values *for each individual track* so you could presumably trim individual drives to avoid bad areas on a platter.
of course it controls the head position regardless of the physical platter, so it wouldn't really be helpful unless the drive only had a single platter in it. i've never seen an actual chip soldered in here and it was completely removed in later board revisions, so i guess they never fully implemented it.