The APC III was
not IBM compatible at all in it's original form. It had a
special version of DOS and the floppy drives are _Quad_ density not double
density.
Not at all??? It's got an AMD 8086 and a Intel 8087 directly on the
motherboard.
About the only thing that I can see that is non-standard is the bus
archetecture. It's got a standard (looking) floppy, and an MFM (or RLL)
HD.
Having familiar looking hardware doesn't mean it is IBM compatible. It was
the bios that counted in those days. NEC did it's own thing here rather than
go to Phoenix.