That sounds like a safe bet according to my book (MacWEEK Upgrading and repairing your mac). The tables are a little hard to decipher, but i'm guessing it's a class code 4 (Memory Test - Mod3 Test) which "Identifies bad chips". Possibly: Data Bit 10, Location G7. Sub Code Bits 0400 if that makes any sense to anybody? It looks like an address error (0002 Address Error)? On this page, http://en.wikipedia.org/wiki/Macintosh_128K, there's a motherboard picture for the 128K -> http://en.wikipedia.org/wiki/File:Macintosh-motherboard.jpg. It looks like, from my limited knowledge, you should replace the chip that is located at Column G, Row 7 in the picture. The chip has the labels '8428' and 'MT4264' on it in the picture.