If it is duff, that's a shame - it was definitely OK when I had it (correct checksum, etc) but they are notoriously fragile. That's the one where the pin had broken off on the corner, isn't it, so maybe it's better in the long run to replace it anyway.
The file you would be looking for for UD6 is the one named thusly:-
basic-2-c000.901465-01.bin2009-08-18
under 'Files' on this page:-
http://www.zimmers.net/anonftp/pub/c...pet/index.html
UD7 which you've already replaced should have been programmed with the file named:-
basic-2-d000.901465-02.bin2009-08-18
If you are wondering how I know which code goes where, look closely at the circuit diagram of the PROMs, which shows the address range occupied by each PROM. UD6 = C000 onwards, UD7 = D000 onwards. Those two hexadecimal addresses are included as part of the file names above.