Stu, are the rear wheels on later models are slightly wider than the early models? Either way, early and later models have specific wheel locations.
Ross, you're right, the wheels on the ISF can only be fitted to specific corners; I've seen them swapped from left to right which just means the cooling effect of the spokes is lost, but I've never seen them swapped corner to corner meaning that one of the back wheels is a 225 and the other is a 255 - far from ideal if it's not spotted.