That much larger difference is what I was looking for. I'm afraid it seems you need at least two replacement MC3446 ICs. You may eventually need a third, although if you could get UA9 out in one piece you would be able to swap it with the replacement UA7 / UA8, as per Owen's suggestion, in order to check it using your test program.
Just got a response from John over in his thread, which you may have seen - the parts he used to repair his IEEE interface were sold as MC3446 equivalents and were marked (Motorola logo) 1820-1689.
I found this useful list of Hewlett-Packard part numbers of ICs which for reasons best known to them, they rebranded with their own part numbers.
http://matthieu.benoit.free.fr/cross...nt/hp-part.htm
If you do a CTRL-F on that page and search 1820-1689 you will find that was HP's part number for their rebranded version of the Motorola MC3446.
The first thing to say is that you don't actually have to fix this fault unless you plan on using the IEEE port with an SD2PET or an original Commodore disc drive if you can find one.
If you want the machine to be completely working (I know I would) then I would try hitting up Cricklewood to see if they really do have them at the advertised price, as that would be a good price if they do.