Hi. What do you mean by "tracking the 'Proof of Purchase'"? Did you buy lots of power supplies from many vendors? Or possibly this power supply was just part of the computer case from Coolermaster, like Elite 342, for example? Usually, the power supply that comes within the case has 2 different stickers with numbers: one shows attachment to specific case that it's part of and another one is the actual serial number of the power supply itself (and it would have the wattage number usually after 2 letters RS... which in your case should look like RS400...).
Technically, you don't need proof of purchase to create RMA - they just use it to determine your warranty start date. You need to register your product first and if you don't provide the proof of purchase - it's OK - they will just have to determine warranty based on your S/N and their internal database which might actually not benefit you after all, especially if you bought it recently. Also, your proof of purchase does not have to contain the serial number - just the model of power supply or chassis that it's part of. Hope this helps!