I don't know about Inside Macintosh, but Macintosh Family Hardware Reference indicates there may have been 3 power supplies. US & International versions in 128K/512K and switchable voltage universal supplies in the Mac Plus: about which it says the input voltage range is configured by closing switch SW1 on the analogue board for 120 V operation. Now correct me if I'm wrong but isn't SW1 the power switch? Either way, I seem to recall reading somewhere that the MacPlus (or at least later models) incorporated a universal power supply, while the older Macs/models were 120 only or both. The main thing is to determine which board he has and it's possible some later 512Ks had the dual board and not to preclude the possibility the original was replaced with a newer universal. He'll need to crack it open and get the part number for the analogue board for starters.