If you search the forum you'll find several long threads about the innards of the HD-20, and I'm pretty sure more than one of them link to a Flickr set of a fairly detailed teardown... here's a link to the start.
Looking at it again, well... I am sort of wondering if you might have a point. At least the mechanical mechanism in the HD-20 was manufactured by Rodime, and there are similarities in the external case styling... perhaps Rodime was a full OEM partner with Apple on the HD-20? A look inside yours might actually *be* interesting. I suppose it's not *impossible* that it could use the same drive with a SCSI bridge in front of it in place of the IWM controller board.