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I wouldn't worry about which 3, isolate them all. To find out I'd have to dismantle mine. As I used M5 nuts on m4 bolts it was a fiddly task to get them fitted. I'd recommend using M4 standoffs if you can get some, failing that use the same m4 bolts with m4 nuts, a lot easier to fit :-). Avoid using shorter bolts because that means you will be blocking the air vents on the underside of the power supply. ... Stu