what chip is that? IBM Displaywriter 6360 Diskette Unit

We've had some success decoding the various LSI chips found within the IBM Displaywriter System, in this case the external 8-inch floppy drive subsystem (IBM 6360), but one last chip has stumped us, or more accurately stumped our Estonian colleague who was familiar with the IBM to Intel part number mapping, see here: http://www.cpu-world.com/forum/viewtopic.php?p=164045#164045 Anyone with knowledge of how to implement the NEC D765D floppy disk controller in a detached setup might be able to make a good guess as to the function of the remaining 40-pin DIP IC (labelled 4430030) in the picture? http://i.imgur.com/VtMxSqj.jpg is it another peripheral chip or perhaps a CPU? I'm guessing the former since I don't see anything that might hold ROM code for a CPU. The Intel 4178628 to the right is an Intel 8255A-5 (Programmable Peripheral Interface (PPI)).