Help identifying mac plus upgrade pcb

[Andrew]

a-HA.  Well spotted.  I was too tired last night to do mental arithmetic and couldn't work out why the checksum hadn't changed.

I really can't understand why someone could go into all that trouble just to patch one or two bytes.

Looking at the source code: https://www.bigmessowires.com/rom-adapter/plus-rom-listing.asm , it doesn't even seem to be executable code at that address. It looks more like a table of characters. I believe that the pcb:

a) Was installed by the greek apple distributor, in order to fix some kind of greek bug

or

b) was installed as part of some anti-privacy mechanism for specialised software (kind of like an old school usb-dongle)

Could it be that the mystery ic is used to inject data from the add-on eeprom, only when triggered somehow? E.g. when certain rom address is accessed in a specific pattern?

 

[cheesestraws]

You'd still only be able to alter every other byte, which would seem ... the very hard way of doing that.  To my mind, it doesn't look like there's enough logic on there to do that, either, but I may be wrong...