I was looking at my 6128+ today which has an MCO122A mainboard and noticed it had a couple of differences from the pictures of the boards on the wiki. There appears to be a couple of resistors or maybe diode on the 74HCT02 IC and there is a jumper from an empty capacitor to one of the memory chips and there is an extra capacitor jumping to this too. I assume they had some sort of memory issue with the computer which required this mod. I notice the resistors on the 74HCT02 are there on some of the 464+ mainboards. I'll have to try getting some better pictures as this version of the mainboard doesn't seem to be on the wiki.
The jumpers are on every 6128+ as far as I know. They seem to be solving a ground loop issue. I've never tried, but my guess is that the disk drive becomes unstable if you remove them. I don't think they solve any RAM issue, they just happened to use the ground pin of the RAM IC because it was easy to access from above.
I've never looked into the other bodged resistors / diodes, so I've no idea what they are solving.
Bryce.