Peter,
The pic I attached that is the standard case fan plug shows two outer raised plastic ridges, one next to the hole for pin 1 and the other next to the hole for pin 4. To get your fan's plug to look similar you want to shave off the ridge that is central... not the one on the edge. I have no idea why you wrote "I suppose that once the outer ridge is removed..." because I specifically said the central ridge was to be removed, not the outer ridge. You can look up the pin/wire info for standard PWM plug ends... they're the same wire order if you have at least one outer ridge for reference. Pretend you're a motherboard header looking up at the plug coming down at you. You might want to have a good computer shop help you locally?