Whats this mean ?

[BrianG]

Those U, D, Q, C, R, L, etc markings would be printed on the PCB, not the device. And in a PCB as cramped as the MMM, I bet those markings are under the components, especially the ICs. Caps, diodes, resistors, and other smaller components may have the letter/number next to it.

At any rate, I just looked at an old MMM brains board pic and cannot see any of those device numbers (don't mind the circles/arrow - it came from the BEC mod thread):



So, likely there is a schematic diagram and/or a layout diagram with those letter/number designations on there. Unless you have those documents, figuring out what was replaced will be next to impossible unless you know what part burnt/fell off when you sent it in.